diff --git a/src/client/render/gl/RenderSettings.ts b/src/client/render/gl/RenderSettings.ts
index 227e5f502..f3387defe 100644
--- a/src/client/render/gl/RenderSettings.ts
+++ b/src/client/render/gl/RenderSettings.ts
@@ -68,6 +68,7 @@ export interface RenderSettings {
   mapOverlay: {
     trailAlpha: number;
     defenseCheckerDarken: number;
+    territoryDefenseDarken: number;
     staleNukeBase: number;
     staleNukeVariation: number;
     staleNukeAlpha: number;
diff --git a/src/client/render/gl/Renderer.ts b/src/client/render/gl/Renderer.ts
index f54e3660c..8ea85c237 100644
--- a/src/client/render/gl/Renderer.ts
+++ b/src/client/render/gl/Renderer.ts
@@ -33,6 +33,7 @@ import { BorderComputePass } from "./passes/BorderComputePass";
 import { BorderStampPass } from "./passes/BorderStampPass";
 import { CoordinateGridPass } from "./passes/CoordinateGridPass";
 import { CrosshairPass } from "./passes/CrosshairPass";
+import { DefenseCoveragePass } from "./passes/DefenseCoveragePass";
 import { FalloutBloomPass } from "./passes/FalloutBloomPass";
 import { FalloutLightPass } from "./passes/FalloutLightPass";
 import { FxPass } from "./passes/fx-pass";
@@ -101,6 +102,7 @@ export class GPURenderer {
   private trailPass: TrailPass;
   private borderStampPass: BorderStampPass;
   private borderPass: BorderComputePass;
+  private defenseCoveragePass: DefenseCoveragePass;
   private bloomPass: FalloutBloomPass;
   private pointLightPass: PointLightPass;
   private falloutLightPass: FalloutLightPass;
@@ -308,6 +310,17 @@ export class GPURenderer {
     );
     this.res.borderTex = this.borderPass.getBorderTex();
 
+    // --- Defense coverage (needs tileTex) — per-tile "defended by same-owner
+    // post" flag, stamped one instanced circle per post. Replaces the old
+    // 64-cap uniform loop; consumed by BorderStampPass. ---
+    this.defenseCoveragePass = new DefenseCoveragePass(
+      gl,
+      mapW,
+      mapH,
+      this.res.tileTex,
+      this.settings,
+    );
+
     // --- Heat manager (needs tileTex, heatTexA/B) ---
     this.heatManager = new HeatManager(
       gl,
@@ -334,10 +347,19 @@ export class GPURenderer {
       this.settings,
     );
     // Route per-tile changes to the border pass so it can scatter-recompute
-    // just the affected tiles instead of rebuilding the whole map.
-    this.territoryPass.setBorderPatchConsumer((x, y) =>
-      this.borderPass.patchTile(x, y),
+    // just the affected tiles instead of rebuilding the whole map. A tile
+    // changing owner can also flip its defense-coverage flag (same-owner test),
+    // so mark the coverage stale too — one coalesced re-stamp happens per frame.
+    this.territoryPass.setBorderPatchConsumer((x, y) => {
+      this.borderPass.patchTile(x, y);
+      this.defenseCoveragePass.markTileDirty(x, y);
+    });
+    // Territory fill darkens on interior tiles defended by a same-owner post;
+    // borderTex lets the fill skip border tiles (those get the checkerboard).
+    this.territoryPass.setDefenseCoverageTex(
+      this.defenseCoveragePass.getCoverageTex(),
     );
+    this.territoryPass.setBorderTex(this.res.borderTex);
 
     // --- Spawn overlay (needs tileTex) ---
     this.spawnOverlayPass = new SpawnOverlayPass(
@@ -368,6 +390,9 @@ export class GPURenderer {
       this.res.borderTex,
       this.settings,
     );
+    this.borderStampPass.setDefenseCoverageTex(
+      this.defenseCoveragePass.getCoverageTex(),
+    );
 
     // --- Fallout bloom (needs tileTex, heatManager) ---
     this.bloomPass = new FalloutBloomPass(
@@ -822,7 +847,7 @@ export class GPURenderer {
         });
       }
     }
-    this.borderPass.updateDefensePosts(posts);
+    this.defenseCoveragePass.updateDefensePosts(posts);
   }
 
   applyDeadUnits(deadUnits: DeadUnitFx[]): void {
@@ -1203,6 +1228,7 @@ export class GPURenderer {
     // pushed per-tile border patches via the wired `borderPatchConsumer`.
     if (this.territoryPass.flushTileTexture() === "full") {
       this.borderPass.markGlobalDirty();
+      this.defenseCoveragePass.markDirty();
     }
     this.trailPass.flushTexture();
     this.heatManager.updateHeat();
@@ -1210,6 +1236,9 @@ export class GPURenderer {
 
   private computeTextures(): void {
     if (this.settings.passEnabled.borderCompute) this.borderPass.draw();
+    // Re-stamp defense coverage if posts/territory changed (dirty-gated).
+    // Leaves the default framebuffer bound; renderFrame resets the viewport.
+    this.defenseCoveragePass.draw();
   }
 
   private renderFrame(): void {
@@ -1328,6 +1357,7 @@ export class GPURenderer {
     this.trailPass.dispose();
     this.borderStampPass.dispose();
     this.borderPass.dispose();
+    this.defenseCoveragePass.dispose();
     this.bloomPass.dispose();
     this.pointLightPass.dispose();
     this.falloutLightPass.dispose();
diff --git a/src/client/render/gl/debug/Layout.ts b/src/client/render/gl/debug/Layout.ts
index 3e9074c2a..66ebef676 100644
--- a/src/client/render/gl/debug/Layout.ts
+++ b/src/client/render/gl/debug/Layout.ts
@@ -122,6 +122,7 @@ export function buildTree(s: RenderSettings, d: RenderSettings): DebugNode[] {
     folder("Map Overlay", [
       slider(s.mapOverlay, "trailAlpha", d.mapOverlay, 0, 1, 0.01),
       slider(s.mapOverlay, "defenseCheckerDarken", d.mapOverlay, 0, 1, 0.01),
+      slider(s.mapOverlay, "territoryDefenseDarken", d.mapOverlay, 0, 1, 0.01),
       slider(s.mapOverlay, "staleNukeBase", d.mapOverlay, 0, 0.3, 0.005),
       slider(s.mapOverlay, "staleNukeVariation", d.mapOverlay, 0, 0.3, 0.005),
       slider(s.mapOverlay, "staleNukeAlpha", d.mapOverlay, 0, 1, 0.01),
diff --git a/src/client/render/gl/passes/BorderComputePass.ts b/src/client/render/gl/passes/BorderComputePass.ts
index 26130cc8f..414eb7ddf 100644
--- a/src/client/render/gl/passes/BorderComputePass.ts
+++ b/src/client/render/gl/passes/BorderComputePass.ts
@@ -5,7 +5,7 @@
  * RGBA8 texture:
  *   R = border type: 0 = interior, 0.5 = normal border, 1.0 = highlight border
  *   G = unused (was ember intensity — moved to FalloutBloomPass/FalloutLightPass)
- *   B = defense proximity: 1.0 if border tile is within range of same-owner defense post
+ *   B = unused (was defense proximity — now computed per-tile by DefenseCoveragePass)
  *
  * Both MapOverlayPass (daytime) and the night stamp overlay read this buffer
  * instead of independently computing neighbor checks. Border thickening is
@@ -24,8 +24,6 @@ import {
 import { TILE_DEFINES } from "../utils/TileCodec";
 import { BorderScatterPass } from "./BorderScatterPass";
 
-const MAX_DEFENSE_POSTS = 64;
-
 /** Max player smallID supported by the relationship texture. */
 const RELATION_TEX_SIZE = 1024;
 
@@ -49,22 +47,15 @@ export class BorderComputePass {
   private uMapSize: WebGLUniformLocation;
   private uHighlightOwner: WebGLUniformLocation;
   private uHighlightThicken: WebGLUniformLocation;
-  private uDefensePosts: WebGLUniformLocation;
-  private uDefensePostCount: WebGLUniformLocation;
-  private uDefensePostRange: WebGLUniformLocation;
 
   private highlightOwner = 0;
   /**
    * True when something that affects ALL borders (highlight owner, relation
-   * matrix, defense posts) has changed since the last draw. Forces a full
-   * recompute next frame. Starts true so the first frame computes.
+   * matrix) has changed since the last draw. Forces a full recompute next
+   * frame. Starts true so the first frame computes.
    */
   private globalDirty = true;
 
-  /** Packed defense post data: [x, y, ownerID, 0, x, y, ownerID, 0, ...] */
-  private defensePostData = new Float32Array(MAX_DEFENSE_POSTS * 4);
-  private defensePostCount = 0;
-
   /** Incremental per-tile recompute. Used between full recomputes. */
   private scatter!: BorderScatterPass;
 
@@ -83,7 +74,7 @@ export class BorderComputePass {
     this.program = createProgram(
       gl,
       fullscreenNoUvVertSrc,
-      shaderSrc(borderComputeFragSrc, { ...TILE_DEFINES, MAX_DEFENSE_POSTS }),
+      shaderSrc(borderComputeFragSrc, { ...TILE_DEFINES }),
     );
 
     this.uMapSize = gl.getUniformLocation(this.program, "uMapSize")!;
@@ -95,15 +86,6 @@ export class BorderComputePass {
       this.program,
       "uHighlightThicken",
     )!;
-    this.uDefensePosts = gl.getUniformLocation(this.program, "uDefensePosts")!;
-    this.uDefensePostCount = gl.getUniformLocation(
-      this.program,
-      "uDefensePostCount",
-    )!;
-    this.uDefensePostRange = gl.getUniformLocation(
-      this.program,
-      "uDefensePostRange",
-    )!;
 
     // Texture unit binding
     gl.useProgram(this.program);
@@ -195,23 +177,6 @@ export class BorderComputePass {
     this.globalDirty = true;
   }
 
-  /** Update defense post positions for checkerboard proximity. */
-  updateDefensePosts(posts: { x: number; y: number; ownerID: number }[]): void {
-    const count = Math.min(posts.length, MAX_DEFENSE_POSTS);
-    const data = this.defensePostData;
-    for (let i = 0; i < count; i++) {
-      const p = posts[i];
-      const off = i * 4;
-      data[off] = p.x;
-      data[off + 1] = p.y;
-      data[off + 2] = p.ownerID;
-      data[off + 3] = 0;
-    }
-    this.defensePostCount = count;
-    this.scatter.setDefensePostData(data, count);
-    this.globalDirty = true;
-  }
-
   /**
    * Force a full recompute next draw. Use this when tile state has been
    * replaced wholesale (initial load, seek) — individual `patchTile` calls
@@ -238,8 +203,8 @@ export class BorderComputePass {
 
   /**
    * Update border flags for the current frame. Either a full recompute (when
-   * globalDirty is set by highlight/relation/defense-post changes) or a
-   * scatter of the per-tile patches queued via `patchTile`.
+   * globalDirty is set by highlight/relation changes) or a scatter of the
+   * per-tile patches queued via `patchTile`.
    *
    * Exit GL state:
    *   - Full recompute path: `borderFbo` is still bound; viewport at map size.
@@ -263,9 +228,6 @@ export class BorderComputePass {
       gl.uniform2f(this.uMapSize, this.mapW, this.mapH);
       gl.uniform1ui(this.uHighlightOwner, this.highlightOwner);
       gl.uniform1i(this.uHighlightThicken, Math.floor(mo.highlightThicken));
-      gl.uniform4fv(this.uDefensePosts, this.defensePostData);
-      gl.uniform1i(this.uDefensePostCount, this.defensePostCount);
-      gl.uniform1f(this.uDefensePostRange, mo.defensePostRange);
 
       gl.activeTexture(gl.TEXTURE0);
       gl.bindTexture(gl.TEXTURE_2D, this._tileTex);
diff --git a/src/client/render/gl/passes/BorderScatterPass.ts b/src/client/render/gl/passes/BorderScatterPass.ts
index 9e5d0429a..65851e125 100644
--- a/src/client/render/gl/passes/BorderScatterPass.ts
+++ b/src/client/render/gl/passes/BorderScatterPass.ts
@@ -26,7 +26,6 @@ import { TILE_DEFINES } from "../utils/TileCodec";
 import borderComputeFragSrc from "../shaders/border-compute/border-compute.frag.glsl?raw";
 import borderScatterVertSrc from "../shaders/border-compute/border-scatter.vert.glsl?raw";
 
-const MAX_DEFENSE_POSTS = 64;
 const FLOATS_PER_PATCH = 2;
 const INITIAL_CAPACITY = 4096;
 
@@ -42,9 +41,6 @@ export class BorderScatterPass {
   private uMapSize: WebGLUniformLocation;
   private uHighlightOwner: WebGLUniformLocation;
   private uHighlightThicken: WebGLUniformLocation;
-  private uDefensePosts: WebGLUniformLocation;
-  private uDefensePostCount: WebGLUniformLocation;
-  private uDefensePostRange: WebGLUniformLocation;
 
   private fbo: WebGLFramebuffer;
   private vao: WebGLVertexArrayObject;
@@ -52,8 +48,6 @@ export class BorderScatterPass {
 
   // Mirrored from BorderComputePass — set via setters when those change.
   private highlightOwner = 0;
-  private defensePostData = new Float32Array(MAX_DEFENSE_POSTS * 4);
-  private defensePostCount = 0;
 
   /** CPU-side patch buffer: [x, y, x, y, …]. */
   private patchData: Float32Array;
@@ -80,7 +74,7 @@ export class BorderScatterPass {
     this.program = createProgram(
       gl,
       borderScatterVertSrc,
-      shaderSrc(borderComputeFragSrc, { ...TILE_DEFINES, MAX_DEFENSE_POSTS }),
+      shaderSrc(borderComputeFragSrc, { ...TILE_DEFINES }),
     );
 
     this.uMapSize = gl.getUniformLocation(this.program, "uMapSize")!;
@@ -92,15 +86,6 @@ export class BorderScatterPass {
       this.program,
       "uHighlightThicken",
     )!;
-    this.uDefensePosts = gl.getUniformLocation(this.program, "uDefensePosts")!;
-    this.uDefensePostCount = gl.getUniformLocation(
-      this.program,
-      "uDefensePostCount",
-    )!;
-    this.uDefensePostRange = gl.getUniformLocation(
-      this.program,
-      "uDefensePostRange",
-    )!;
 
     gl.useProgram(this.program);
     gl.uniform1i(gl.getUniformLocation(this.program, "uTileTex"), 0);
@@ -158,12 +143,6 @@ export class BorderScatterPass {
     this.highlightOwner = owner;
   }
 
-  setDefensePostData(data: Float32Array, count: number): void {
-    // Caller may mutate the source array; copy to keep ours stable.
-    this.defensePostData.set(data.subarray(0, MAX_DEFENSE_POSTS * 4));
-    this.defensePostCount = count;
-  }
-
   flush(): void {
     if (this.patchCount === 0) return;
     const gl = this.gl;
@@ -190,9 +169,6 @@ export class BorderScatterPass {
     gl.uniform2f(this.uMapSize, this.mapW, this.mapH);
     gl.uniform1ui(this.uHighlightOwner, this.highlightOwner);
     gl.uniform1i(this.uHighlightThicken, Math.floor(mo.highlightThicken));
-    gl.uniform4fv(this.uDefensePosts, this.defensePostData);
-    gl.uniform1i(this.uDefensePostCount, this.defensePostCount);
-    gl.uniform1f(this.uDefensePostRange, mo.defensePostRange);
 
     gl.activeTexture(gl.TEXTURE0);
     gl.bindTexture(gl.TEXTURE_2D, this.tileTex);
diff --git a/src/client/render/gl/passes/BorderStampPass.ts b/src/client/render/gl/passes/BorderStampPass.ts
index c3925e00f..8f1ce474c 100644
--- a/src/client/render/gl/passes/BorderStampPass.ts
+++ b/src/client/render/gl/passes/BorderStampPass.ts
@@ -35,6 +35,7 @@ export class BorderStampPass {
   private tileTex: WebGLTexture;
   private paletteTex: WebGLTexture;
   private borderTex: WebGLTexture;
+  private defenseCoverageTex: WebGLTexture | null = null;
   private affiliationTex: WebGLTexture | null = null;
   private altView = false;
 
@@ -90,6 +91,7 @@ export class BorderStampPass {
     gl.uniform1i(gl.getUniformLocation(this.program, "uPalette"), 1);
     gl.uniform1i(gl.getUniformLocation(this.program, "uBorderTex"), 2);
     gl.uniform1i(gl.getUniformLocation(this.program, "uAffiliation"), 3);
+    gl.uniform1i(gl.getUniformLocation(this.program, "uDefenseCoverageTex"), 4);
 
     this.vao = createMapQuad(gl, mapW, mapH);
   }
@@ -100,6 +102,9 @@ export class BorderStampPass {
   setAffiliationTex(tex: WebGLTexture): void {
     this.affiliationTex = tex;
   }
+  setDefenseCoverageTex(tex: WebGLTexture): void {
+    this.defenseCoverageTex = tex;
+  }
 
   /** Draw borders + defense checkerboard. Blending must be enabled. */
   draw(cameraMatrix: Float32Array): void {
@@ -137,6 +142,10 @@ export class BorderStampPass {
       gl.activeTexture(gl.TEXTURE3);
       gl.bindTexture(gl.TEXTURE_2D, this.affiliationTex);
     }
+    if (this.defenseCoverageTex) {
+      gl.activeTexture(gl.TEXTURE4);
+      gl.bindTexture(gl.TEXTURE_2D, this.defenseCoverageTex);
+    }
 
     gl.bindVertexArray(this.vao);
     gl.drawArrays(gl.TRIANGLES, 0, 6);
diff --git a/src/client/render/gl/passes/DefenseCoveragePass.ts b/src/client/render/gl/passes/DefenseCoveragePass.ts
new file mode 100644
index 000000000..f7593dbef
--- /dev/null
+++ b/src/client/render/gl/passes/DefenseCoveragePass.ts
@@ -0,0 +1,280 @@
+/**
+ * DefenseCoveragePass — per-tile "is this tile defended by a same-owner Defense
+ * Post?" flag, computed by stamping one instanced circle per post.
+ *
+ * Replaces the old per-fragment scan (border-compute looped over a uniform array
+ * of up to 64 posts for every border tile). Here we invert the loop: each post
+ * draws a filled circle of its range into a map-resolution R8 texture, writing
+ * 1.0 on tiles it owns and within range. Cost is O(posts × circle area) with no
+ * cap on post count, and it's a single instanced draw call regardless of how
+ * many posts exist — the same pattern UnitPass/StructurePass already use.
+ *
+ * BorderStampPass samples the resulting `coverageTex` (one texel per border
+ * tile) instead of the old uniform loop. The texture marks every same-owner
+ * in-range tile, interior included — so a future PR can darken the territory
+ * fill by sampling the same texture in TerritoryPass.
+ *
+ * The result depends on tile ownership (the same-owner test), so coverage must
+ * be re-stamped whenever posts OR territory change. Territory drips every frame
+ * during combat, so a full map re-stamp every frame would be wasteful at high
+ * post counts. Instead we track a grid of dirty BLOCKs: a tile changing owner
+ * only changes its own coverage, so we recompute just the blocks containing
+ * changed tiles, scissored to each block (clear the block, redraw the posts —
+ * the scissor confines the GPU work to the changed region). A post add/remove
+ * or full tile upload sets `fullDirty` for one whole-map stamp; if too many
+ * blocks are dirty we fall back to a full stamp too.
+ *
+ * Exit GL state: default framebuffer bound; viewport left at map size; scissor
+ * test disabled. The caller (Renderer.renderFrame) rebinds framebuffer +
+ * viewport before screen draws.
+ */
+
+import { DynamicInstanceBuffer } from "../DynamicBuffer";
+import type { RenderSettings } from "../RenderSettings";
+import coverageFragSrc from "../shaders/defense-coverage/defense-coverage.frag.glsl?raw";
+import coverageVertSrc from "../shaders/defense-coverage/defense-coverage.vert.glsl?raw";
+import { createProgram, createTexture2D, shaderSrc } from "../utils/GlUtils";
+import { TILE_DEFINES } from "../utils/TileCodec";
+
+/** Per-instance data (3 floats): tileX, tileY, ownerID. */
+const FLOATS_PER_INSTANCE = 3;
+
+/**
+ * Tile block size for incremental scissored re-stamping. ~2× the post diameter
+ * (range ≈ 30, so circles ≈ 60 wide): small enough to confine work to the
+ * changed region, large enough to keep the per-block draw-call count low.
+ */
+const BLOCK = 128;
+
+export class DefenseCoveragePass {
+  private gl: WebGL2RenderingContext;
+  private settings: RenderSettings;
+  private mapW: number;
+  private mapH: number;
+  private tileTex: WebGLTexture;
+
+  private program: WebGLProgram;
+  private uMapSize: WebGLUniformLocation;
+  private uRange: WebGLUniformLocation;
+
+  private coverageTex: WebGLTexture;
+  private fbo: WebGLFramebuffer;
+
+  private quadBuf: WebGLBuffer;
+  private vao: WebGLVertexArrayObject;
+  private instanceBuf: DynamicInstanceBuffer;
+  private count = 0;
+
+  // --- Dirty tracking (block grid) ---
+  private blocksX: number;
+  private blocksY: number;
+  /** Re-stamp the whole map next draw. Starts true so the first frame computes. */
+  private fullDirty = true;
+  /** Per-block dirty flag (0/1), indexed by blockY * blocksX + blockX. */
+  private dirtyBlock: Uint8Array;
+  /** Indices of currently-dirty blocks (to iterate + reset without scanning). */
+  private dirtyList: number[] = [];
+  /** Above this many dirty blocks, a single full stamp is cheaper. */
+  private fullFallback: number;
+
+  constructor(
+    gl: WebGL2RenderingContext,
+    mapW: number,
+    mapH: number,
+    tileTex: WebGLTexture,
+    settings: RenderSettings,
+  ) {
+    this.gl = gl;
+    this.settings = settings;
+    this.mapW = mapW;
+    this.mapH = mapH;
+    this.tileTex = tileTex;
+
+    this.blocksX = Math.ceil(mapW / BLOCK);
+    this.blocksY = Math.ceil(mapH / BLOCK);
+    this.dirtyBlock = new Uint8Array(this.blocksX * this.blocksY);
+    // Past ~half the blocks, one full stamp beats many scissored block draws.
+    this.fullFallback = Math.floor((this.blocksX * this.blocksY) / 2);
+
+    this.program = createProgram(
+      gl,
+      coverageVertSrc,
+      shaderSrc(coverageFragSrc, { OWNER_MASK: TILE_DEFINES.OWNER_MASK }),
+    );
+    this.uMapSize = gl.getUniformLocation(this.program, "uMapSize")!;
+    this.uRange = gl.getUniformLocation(this.program, "uRange")!;
+
+    gl.useProgram(this.program);
+    gl.uniform1i(gl.getUniformLocation(this.program, "uTileTex"), 0);
+
+    // --- R8 coverage texture at tile resolution + its FBO ---
+    this.coverageTex = createTexture2D(gl, {
+      width: mapW,
+      height: mapH,
+      internalFormat: gl.R8,
+      format: gl.RED,
+      type: gl.UNSIGNED_BYTE,
+      data: null,
+      filter: gl.NEAREST,
+    });
+    this.fbo = gl.createFramebuffer()!;
+    gl.bindFramebuffer(gl.FRAMEBUFFER, this.fbo);
+    gl.framebufferTexture2D(
+      gl.FRAMEBUFFER,
+      gl.COLOR_ATTACHMENT0,
+      gl.TEXTURE_2D,
+      this.coverageTex,
+      0,
+    );
+    gl.bindFramebuffer(gl.FRAMEBUFFER, null);
+
+    // --- Shared unit quad [0,1]² ---
+    this.quadBuf = gl.createBuffer()!;
+    gl.bindBuffer(gl.ARRAY_BUFFER, this.quadBuf);
+    gl.bufferData(
+      gl.ARRAY_BUFFER,
+      new Float32Array([0, 0, 1, 0, 0, 1, 1, 0, 1, 1, 0, 1]),
+      gl.STATIC_DRAW,
+    );
+
+    // --- Per-post instance buffer + VAO ---
+    const instGlBuf = gl.createBuffer()!;
+    this.instanceBuf = new DynamicInstanceBuffer(
+      gl,
+      instGlBuf,
+      256,
+      FLOATS_PER_INSTANCE,
+    );
+    this.vao = gl.createVertexArray()!;
+    gl.bindVertexArray(this.vao);
+    gl.bindBuffer(gl.ARRAY_BUFFER, this.quadBuf);
+    gl.enableVertexAttribArray(0);
+    gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0);
+    gl.bindBuffer(gl.ARRAY_BUFFER, instGlBuf);
+    gl.enableVertexAttribArray(1);
+    gl.vertexAttribPointer(1, 3, gl.FLOAT, false, FLOATS_PER_INSTANCE * 4, 0);
+    gl.vertexAttribDivisor(1, 1);
+    gl.bindVertexArray(null);
+  }
+
+  /** Replace the set of defense posts. No cap. */
+  updateDefensePosts(posts: { x: number; y: number; ownerID: number }[]): void {
+    this.count = posts.length;
+    this.instanceBuf.ensureCapacity(posts.length);
+    const f = this.instanceBuf.float32;
+    for (let i = 0; i < posts.length; i++) {
+      const p = posts[i];
+      const off = i * FLOATS_PER_INSTANCE;
+      f[off] = p.x;
+      f[off + 1] = p.y;
+      f[off + 2] = p.ownerID;
+    }
+    if (posts.length > 0) {
+      const gl = this.gl;
+      gl.bindBuffer(gl.ARRAY_BUFFER, this.instanceBuf.buffer);
+      gl.bufferSubData(
+        gl.ARRAY_BUFFER,
+        0,
+        this.instanceBuf.float32,
+        0,
+        posts.length * FLOATS_PER_INSTANCE,
+      );
+    }
+    // A post appearing/disappearing affects its whole circle (possibly several
+    // blocks); post-set changes are rare, so just re-stamp the whole map.
+    this.fullDirty = true;
+  }
+
+  /**
+   * Mark the block containing tile (x, y) stale. Call when a tile changed owner
+   * — the same-owner test means only that tile's own coverage can flip, so just
+   * its block needs recomputing. Coalesced: the block is re-stamped once in the
+   * next draw() regardless of how many of its tiles changed.
+   */
+  markTileDirty(x: number, y: number): void {
+    const bx = (x / BLOCK) | 0;
+    const by = (y / BLOCK) | 0;
+    const b = by * this.blocksX + bx;
+    if (this.dirtyBlock[b] === 0) {
+      this.dirtyBlock[b] = 1;
+      this.dirtyList.push(b);
+    }
+  }
+
+  /** Force a whole-map re-stamp next draw (full tile upload / seek). */
+  markDirty(): void {
+    this.fullDirty = true;
+  }
+
+  /** The R8 coverage texture (1.0 = tile is defended by a same-owner post). */
+  getCoverageTex(): WebGLTexture {
+    return this.coverageTex;
+  }
+
+  /**
+   * Re-stamp coverage if anything changed. Either a whole-map stamp (fullDirty,
+   * or too many blocks dirty) or a scissored clear+stamp per dirty block.
+   *
+   * Exit GL state: default framebuffer bound; scissor test disabled; viewport
+   * left at map size (caller resets before screen draws).
+   */
+  draw(): void {
+    if (!this.fullDirty && this.dirtyList.length === 0) return;
+
+    const gl = this.gl;
+    gl.bindFramebuffer(gl.FRAMEBUFFER, this.fbo);
+    gl.viewport(0, 0, this.mapW, this.mapH);
+    gl.disable(gl.BLEND);
+    gl.clearColor(0, 0, 0, 0);
+
+    // Shared stamp state (uniforms/textures/VAO don't change between blocks).
+    gl.useProgram(this.program);
+    gl.uniform2f(this.uMapSize, this.mapW, this.mapH);
+    gl.uniform1f(this.uRange, this.settings.mapOverlay.defensePostRange);
+    gl.activeTexture(gl.TEXTURE0);
+    gl.bindTexture(gl.TEXTURE_2D, this.tileTex);
+    gl.bindVertexArray(this.vao);
+
+    if (this.fullDirty || this.dirtyList.length > this.fullFallback) {
+      // Whole-map stamp.
+      gl.disable(gl.SCISSOR_TEST);
+      gl.clear(gl.COLOR_BUFFER_BIT);
+      if (this.count > 0)
+        gl.drawArraysInstanced(gl.TRIANGLES, 0, 6, this.count);
+    } else {
+      // Per-block scissored stamp — confines clear + draw to changed regions.
+      gl.enable(gl.SCISSOR_TEST);
+      for (const b of this.dirtyList) {
+        const bx = (b % this.blocksX) * BLOCK;
+        const by = ((b / this.blocksX) | 0) * BLOCK;
+        // Tile coords map 1:1 to FBO pixels (no Y flip), so pass the block rect
+        // straight to scissor, clamping the right/bottom edge blocks to bounds.
+        const bw = Math.min(BLOCK, this.mapW - bx);
+        const bh = Math.min(BLOCK, this.mapH - by);
+        gl.scissor(bx, by, bw, bh);
+        gl.clear(gl.COLOR_BUFFER_BIT);
+        if (this.count > 0) {
+          gl.drawArraysInstanced(gl.TRIANGLES, 0, 6, this.count);
+        }
+      }
+      gl.disable(gl.SCISSOR_TEST);
+    }
+
+    // Reset dirty state.
+    for (const b of this.dirtyList) this.dirtyBlock[b] = 0;
+    this.dirtyList.length = 0;
+    this.fullDirty = false;
+
+    gl.bindFramebuffer(gl.FRAMEBUFFER, null);
+  }
+
+  dispose(): void {
+    const gl = this.gl;
+    gl.deleteProgram(this.program);
+    gl.deleteTexture(this.coverageTex);
+    gl.deleteFramebuffer(this.fbo);
+    gl.deleteBuffer(this.quadBuf);
+    gl.deleteVertexArray(this.vao);
+    this.instanceBuf.dispose();
+  }
+}
diff --git a/src/client/render/gl/passes/TerritoryPass.ts b/src/client/render/gl/passes/TerritoryPass.ts
index 07c75a533..a94a897e6 100644
--- a/src/client/render/gl/passes/TerritoryPass.ts
+++ b/src/client/render/gl/passes/TerritoryPass.ts
@@ -40,6 +40,7 @@ export class TerritoryPass {
   private uHighlightBrighten: WebGLUniformLocation;
   private uShowPatterns: WebGLUniformLocation;
   private uIsTeamMode: WebGLUniformLocation;
+  private uDefenseDarken: WebGLUniformLocation;
   private highlightOwner = 0;
   private isTeamMode = false;
 
@@ -51,6 +52,8 @@ export class TerritoryPass {
   private skinAtlasTex: WebGLTexture;
   private skinLayerTex: WebGLTexture;
   private skinAnchorTex: WebGLTexture;
+  private defenseCoverageTex: WebGLTexture | null = null;
+  private borderTex: WebGLTexture | null = null;
 
   private altView = false;
   private showPatterns = true;
@@ -158,6 +161,10 @@ export class TerritoryPass {
     )!;
     this.uShowPatterns = gl.getUniformLocation(this.program, "uShowPatterns")!;
     this.uIsTeamMode = gl.getUniformLocation(this.program, "uIsTeamMode")!;
+    this.uDefenseDarken = gl.getUniformLocation(
+      this.program,
+      "uDefenseDarken",
+    )!;
 
     gl.useProgram(this.program);
     gl.uniform1i(gl.getUniformLocation(this.program, "uTileTex"), 0);
@@ -167,6 +174,8 @@ export class TerritoryPass {
     gl.uniform1i(gl.getUniformLocation(this.program, "uSkinAtlas"), 4);
     gl.uniform1i(gl.getUniformLocation(this.program, "uSkinLayer"), 5);
     gl.uniform1i(gl.getUniformLocation(this.program, "uSkinAnchor"), 6);
+    gl.uniform1i(gl.getUniformLocation(this.program, "uDefenseCoverageTex"), 7);
+    gl.uniform1i(gl.getUniformLocation(this.program, "uBorderTex"), 8);
 
     this.vao = createMapQuad(gl, mapW, mapH);
 
@@ -411,6 +420,16 @@ export class TerritoryPass {
     this.highlightOwner = ownerID;
   }
 
+  /** Defense-coverage texture (R8) — darkens the fill on defended tiles. */
+  setDefenseCoverageTex(tex: WebGLTexture): void {
+    this.defenseCoverageTex = tex;
+  }
+
+  /** Border flags (RGBA8) — used to skip the defense darken on border tiles. */
+  setBorderTex(tex: WebGLTexture): void {
+    this.borderTex = tex;
+  }
+
   /** Draw territory fill + stale-nuke ground. Blending must be enabled by caller. */
   draw(cameraMatrix: Float32Array): void {
     this.flushTileTexture();
@@ -438,6 +457,7 @@ export class TerritoryPass {
       this.settings.passEnabled.territoryPatterns && this.showPatterns ? 1 : 0,
     );
     gl.uniform1i(this.uIsTeamMode, this.isTeamMode ? 1 : 0);
+    gl.uniform1f(this.uDefenseDarken, mo.territoryDefenseDarken);
 
     gl.activeTexture(gl.TEXTURE0);
     gl.bindTexture(gl.TEXTURE_2D, this.tileTex);
@@ -453,6 +473,14 @@ export class TerritoryPass {
     gl.bindTexture(gl.TEXTURE_2D, this.skinLayerTex);
     gl.activeTexture(gl.TEXTURE6);
     gl.bindTexture(gl.TEXTURE_2D, this.skinAnchorTex);
+    if (this.defenseCoverageTex) {
+      gl.activeTexture(gl.TEXTURE7);
+      gl.bindTexture(gl.TEXTURE_2D, this.defenseCoverageTex);
+    }
+    if (this.borderTex) {
+      gl.activeTexture(gl.TEXTURE8);
+      gl.bindTexture(gl.TEXTURE_2D, this.borderTex);
+    }
 
     gl.bindVertexArray(this.vao);
     gl.drawArrays(gl.TRIANGLES, 0, 6);
diff --git a/src/client/render/gl/render-settings.json b/src/client/render/gl/render-settings.json
index cc98d49b1..6a50b7087 100644
--- a/src/client/render/gl/render-settings.json
+++ b/src/client/render/gl/render-settings.json
@@ -66,6 +66,7 @@
   "mapOverlay": {
     "trailAlpha": 0.588,
     "defenseCheckerDarken": 0.7,
+    "territoryDefenseDarken": 0.85,
     "staleNukeBase": 0,
     "staleNukeVariation": 0.05,
     "staleNukeAlpha": 1,
diff --git a/src/client/render/gl/shaders/border-compute/border-compute.frag.glsl b/src/client/render/gl/shaders/border-compute/border-compute.frag.glsl
index 8d060f9a3..14677d14c 100644
--- a/src/client/render/gl/shaders/border-compute/border-compute.frag.glsl
+++ b/src/client/render/gl/shaders/border-compute/border-compute.frag.glsl
@@ -8,11 +8,6 @@ uniform vec2 uMapSize;
 uniform uint uHighlightOwner;
 uniform int uHighlightThicken; // Chebyshev radius for highlight expansion
 
-// Defense post proximity — (x, y, ownerID, _) per post
-uniform vec4 uDefensePosts[MAX_DEFENSE_POSTS];
-uniform int uDefensePostCount;
-uniform float uDefensePostRange;
-
 out vec4 fragColor;
 
 uint getOwner(ivec2 c) {
@@ -82,26 +77,10 @@ void main() {
     }
   }
 
-  // --- Defense post proximity ---
-  float defenseFlag = 0.0;
-  if (borderType > 0.0 && owner != 0u) {
-    float rangeSq = uDefensePostRange * uDefensePostRange;
-    for (int i = 0; i < MAX_DEFENSE_POSTS; i++) {
-      if (i >= uDefensePostCount) break;
-      vec4 dp = uDefensePosts[i];
-      if (uint(dp.z) != owner) continue;
-      float dx = float(tc.x) - dp.x;
-      float dy = float(tc.y) - dp.y;
-      if (dx * dx + dy * dy <= rangeSq) {
-        defenseFlag = 1.0;
-        break;
-      }
-    }
-  }
-
   // A = relationship: 0.0=neutral, 0.5=friendly, 1.0=embargo
   float relation = float(maxRel) * 0.5;
   // G channel is unused (formerly emberIntensity; ember is now computed in
-  // FalloutBloomPass and FalloutLightPass).
-  fragColor = vec4(borderType, 0.0, defenseFlag, relation);
+  // FalloutBloomPass and FalloutLightPass). B channel is unused (defense post
+  // proximity is now computed per-tile by DefenseCoveragePass).
+  fragColor = vec4(borderType, 0.0, 0.0, relation);
 }
diff --git a/src/client/render/gl/shaders/day-night/border-stamp.frag.glsl b/src/client/render/gl/shaders/day-night/border-stamp.frag.glsl
index 62873a97d..2bf641dbb 100644
--- a/src/client/render/gl/shaders/day-night/border-stamp.frag.glsl
+++ b/src/client/render/gl/shaders/day-night/border-stamp.frag.glsl
@@ -5,6 +5,7 @@ precision highp usampler2D;
 uniform usampler2D uTileTex;
 uniform sampler2D uPalette;
 uniform sampler2D uBorderTex;     // RGBA8 — border flags from BorderComputePass
+uniform sampler2D uDefenseCoverageTex; // R8 — 1.0 = defended by same-owner post
 uniform sampler2D uAffiliation;   // 256×2 RGBA8 — affiliation colors (row 0 = border)
 uniform vec2 uMapSize;
 uniform int uAltView;
@@ -28,7 +29,7 @@ void main() {
   // Read pre-computed border flags from BorderComputePass
   vec4 borderData = texelFetch(uBorderTex, tc, 0);
   float borderType = borderData.r;      // 0=interior, ~0.5=normal, ~1.0=highlight
-  bool defense = borderData.b > 0.5;    // defense post proximity
+  bool defense = texelFetch(uDefenseCoverageTex, tc, 0).r > 0.5; // same-owner defense post nearby
   float relation = borderData.a;        // 0.0=neutral, ~0.5=friendly, ~1.0=embargo
 
   bool isBorder = borderType > 0.25;
diff --git a/src/client/render/gl/shaders/defense-coverage/defense-coverage.frag.glsl b/src/client/render/gl/shaders/defense-coverage/defense-coverage.frag.glsl
new file mode 100644
index 000000000..22a27cbe8
--- /dev/null
+++ b/src/client/render/gl/shaders/defense-coverage/defense-coverage.frag.glsl
@@ -0,0 +1,34 @@
+#version 300 es
+precision highp float;
+precision highp usampler2D;
+
+// Writes 1.0 into the coverage texture for every tile within `uRange` of the
+// post being stamped AND owned by that post's owner. The texture is cleared to
+// 0 first, so a tile no post touches stays 0. Overlapping circles from the same
+// owner compose as a boolean union (1.0 over 1.0 is still 1.0 — no blending);
+// circles from other owners discard at this tile and leave it untouched.
+
+uniform usampler2D uTileTex;  // R16UI — tile state per cell
+uniform vec2 uMapSize;
+uniform float uRange;
+
+flat in vec2 vPostCenter;
+flat in float vOwner;
+
+out vec4 fragColor;
+
+void main() {
+  ivec2 tc = ivec2(gl_FragCoord.xy);
+  if (tc.x >= int(uMapSize.x) || tc.y >= int(uMapSize.y)) discard;
+
+  // Circle test in tile-integer space (matches the old per-fragment loop).
+  float dx = float(tc.x) - vPostCenter.x;
+  float dy = float(tc.y) - vPostCenter.y;
+  if (dx * dx + dy * dy > uRange * uRange) discard;
+
+  // Same-owner test: only the tile's own owner's posts defend it.
+  uint owner = texelFetch(uTileTex, tc, 0).r & uint(OWNER_MASK);
+  if (owner != uint(vOwner)) discard;
+
+  fragColor = vec4(1.0);
+}
diff --git a/src/client/render/gl/shaders/defense-coverage/defense-coverage.vert.glsl b/src/client/render/gl/shaders/defense-coverage/defense-coverage.vert.glsl
new file mode 100644
index 000000000..27102e596
--- /dev/null
+++ b/src/client/render/gl/shaders/defense-coverage/defense-coverage.vert.glsl
@@ -0,0 +1,29 @@
+#version 300 es
+precision highp float;
+
+// One instanced quad per defense post. The quad is a box centered on the post,
+// sized to cover the post's range; the fragment shader trims it to a circle and
+// filters by tile owner. See DefenseCoveragePass.
+
+layout(location = 0) in vec2 aCorner;  // unit quad corner, [0,1]²
+layout(location = 1) in vec3 aPost;     // (tileX, tileY, ownerID)
+
+uniform vec2 uMapSize;
+uniform float uRange;
+
+flat out vec2 vPostCenter;
+flat out float vOwner;
+
+void main() {
+  vPostCenter = aPost.xy;
+  vOwner = aPost.z;
+
+  // Box spanning [center - range, center + range] in tile coords, plus a
+  // 1-tile margin so the boundary tiles at exactly `range` are rasterized
+  // (their pixel centers sit just past the un-padded edge).
+  vec2 tilePos = aPost.xy + (aCorner * 2.0 - 1.0) * (uRange + 1.0);
+
+  // Tile-resolution FBO (viewport = map size), so map straight to clip space.
+  vec2 ndc = (tilePos / uMapSize) * 2.0 - 1.0;
+  gl_Position = vec4(ndc, 0.0, 1.0);
+}
diff --git a/src/client/render/gl/shaders/map-overlay/territory.frag.glsl b/src/client/render/gl/shaders/map-overlay/territory.frag.glsl
index 83ecad40d..5e84fbc48 100644
--- a/src/client/render/gl/shaders/map-overlay/territory.frag.glsl
+++ b/src/client/render/gl/shaders/map-overlay/territory.frag.glsl
@@ -22,6 +22,9 @@ uniform float uStaleNukeAlpha;
 uniform vec3 uStaleNukeColor;
 uniform uint uHighlightOwner;      // 0 = no highlight; otherwise smallID of hovered owner
 uniform float uHighlightBrighten;  // mix amount toward white for highlighted tiles
+uniform sampler2D uDefenseCoverageTex; // R8 — 1.0 = tile defended by same-owner post
+uniform float uDefenseDarken;      // multiplier applied to fill on defended tiles
+uniform sampler2D uBorderTex;      // RGBA8 — border flags; R > 0.25 = border tile
 
 in vec2 vWorldPos;
 out vec4 fragColor;
@@ -108,5 +111,15 @@ void main() {
     color.rgb = clamp(mix(vec3(luma), color.rgb, 1.6), 0.0, 1.0);
   }
 
+  // Defense bonus: darken the fill on interior tiles defended by a same-owner
+  // post. Border tiles are skipped — they get the checkerboard overlay from
+  // BorderStampPass instead. Coverage is tested first so the (rarer) defended
+  // tiles are the only ones that pay for the extra border fetch (&& short-
+  // circuits in GLSL ES 3.00; texelFetch is derivative-free so this is safe).
+  if (texelFetch(uDefenseCoverageTex, tc, 0).r > 0.5 &&
+      texelFetch(uBorderTex, tc, 0).r <= 0.25) {
+    color.rgb *= uDefenseDarken;
+  }
+
   fragColor = color;
 }