diff --git a/src/client/graphics/layers/BuildMenu.ts b/src/client/graphics/layers/BuildMenu.ts
index d7201b9a8..e261b42ad 100644
--- a/src/client/graphics/layers/BuildMenu.ts
+++ b/src/client/graphics/layers/BuildMenu.ts
@@ -16,6 +16,7 @@ import missileSiloIcon from "../../../../resources/images/MissileSiloIconWhite.s
 import goldCoinIcon from "../../../../resources/images/GoldCoinIcon.svg";
 import portIcon from "../../../../resources/images/PortIcon.svg";
 import cityIcon from "../../../../resources/images/CityIconWhite.svg";
+import shieldIcon from "../../../../resources/images/ShieldIconWhite.svg";
 import { renderNumber } from "../../Utils";
 import { GameView, PlayerView } from "../../../core/game/GameView";
 
@@ -31,7 +32,7 @@ const buildTable: BuildItemDisplay[][] = [
     { unitType: UnitType.Warship, icon: warshipIcon },
     { unitType: UnitType.Port, icon: portIcon },
     { unitType: UnitType.MissileSilo, icon: missileSiloIcon },
-    // { unitType: UnitType.DefensePost, icon: shieldIcon },
+    { unitType: UnitType.DefensePost, icon: shieldIcon },
     { unitType: UnitType.City, icon: cityIcon },
   ],
 ];
diff --git a/src/client/graphics/layers/TerritoryLayer.ts b/src/client/graphics/layers/TerritoryLayer.ts
index a0d4e7884..31c1139ac 100644
--- a/src/client/graphics/layers/TerritoryLayer.ts
+++ b/src/client/graphics/layers/TerritoryLayer.ts
@@ -7,7 +7,7 @@ import {
   Unit,
   UnitType,
 } from "../../../core/game/Game";
-import { UnitUpdate } from "../../../core/game/GameUpdates";
+import { GameUpdateType, UnitUpdate } from "../../../core/game/GameUpdates";
 import { PseudoRandom } from "../../../core/PseudoRandom";
 import { colord, Colord } from "colord";
 import { Theme } from "../../../core/configuration/Config";
@@ -19,7 +19,11 @@ import {
   MouseDownEvent,
 } from "../../InputHandler";
 import { GameView, PlayerView } from "../../../core/game/GameView";
-import { euclDistFN, TileRef } from "../../../core/game/GameMap";
+import {
+  euclDistFN,
+  manhattanDistFN,
+  TileRef,
+} from "../../../core/game/GameMap";
 
 export class TerritoryLayer implements Layer {
   private canvas: HTMLCanvasElement;
@@ -46,10 +50,7 @@ export class TerritoryLayer implements Layer {
   private refreshRate = 50;
   private lastRefresh = 0;
 
-  constructor(
-    private game: GameView,
-    private eventBus: EventBus,
-  ) {
+  constructor(private game: GameView, private eventBus: EventBus) {
     this.theme = game.config().theme();
   }
 
@@ -59,6 +60,25 @@ export class TerritoryLayer implements Layer {
 
   tick() {
     this.game.recentlyUpdatedTiles().forEach((t) => this.enqueueTile(t));
+    this.game.updatesSinceLastTick()[GameUpdateType.Unit].forEach((u) => {
+      const update = u as UnitUpdate;
+      if (update.unitType == UnitType.DefensePost && update.isActive) {
+        const tile = this.game.ref(update.pos.x, update.pos.y);
+        this.game
+          .bfs(
+            tile,
+            manhattanDistFN(tile, this.game.config().defensePostRange())
+          )
+          .forEach((t) => {
+            if (
+              this.game.isBorder(t) &&
+              this.game.ownerID(t) == update.ownerID
+            ) {
+              this.enqueueTile(t);
+            }
+          });
+      }
+    });
 
     if (!this.game.inSpawnPhase()) {
       return;
@@ -71,7 +91,7 @@ export class TerritoryLayer implements Layer {
       0,
       0,
       this.game.width(),
-      this.game.height(),
+      this.game.height()
     );
     const humans = this.game
       .playerViews()
@@ -91,7 +111,7 @@ export class TerritoryLayer implements Layer {
           this.paintHighlightCell(
             new Cell(this.game.x(tile), this.game.y(tile)),
             this.theme.spawnHighlightColor(),
-            255,
+            255
           );
         }
       }
@@ -117,7 +137,7 @@ export class TerritoryLayer implements Layer {
       0,
       0,
       this.game.width(),
-      this.game.height(),
+      this.game.height()
     );
     this.initImageData();
     this.canvas.width = this.game.width();
@@ -164,7 +184,7 @@ export class TerritoryLayer implements Layer {
       -this.game.width() / 2,
       -this.game.height() / 2,
       this.game.width(),
-      this.game.height(),
+      this.game.height()
     );
     if (this.game.inSpawnPhase()) {
       context.drawImage(
@@ -172,7 +192,7 @@ export class TerritoryLayer implements Layer {
         -this.game.width() / 2,
         -this.game.height() / 2,
         this.game.width(),
-        this.game.height(),
+        this.game.height()
       );
     }
   }
@@ -203,27 +223,39 @@ export class TerritoryLayer implements Layer {
           this.game.x(tile),
           this.game.y(tile),
           this.theme.falloutColor(),
-          150,
+          150
         );
         return;
       }
       this.clearCell(new Cell(this.game.x(tile), this.game.y(tile)));
       return;
     }
-    const owner = this.game.owner(tile) as Player;
+    const owner = this.game.owner(tile) as PlayerView;
     if (this.game.isBorder(tile)) {
-      this.paintCell(
-        this.game.x(tile),
-        this.game.y(tile),
-        this.theme.borderColor(owner.info()),
-        255,
-      );
+      if (
+        this.game.nearbyDefenses(tile).filter((u) => u.owner() == owner)
+          .length > 0
+      ) {
+        this.paintCell(
+          this.game.x(tile),
+          this.game.y(tile),
+          this.theme.defendedBorderColor(owner.info()),
+          255
+        );
+      } else {
+        this.paintCell(
+          this.game.x(tile),
+          this.game.y(tile),
+          this.theme.borderColor(owner.info()),
+          255
+        );
+      }
     } else {
       this.paintCell(
         this.game.x(tile),
         this.game.y(tile),
         this.theme.territoryColor(owner.info()),
-        150,
+        150
       );
     }
   }
diff --git a/src/core/configuration/Config.ts b/src/core/configuration/Config.ts
index 285b5148d..6f1bae2dc 100644
--- a/src/core/configuration/Config.ts
+++ b/src/core/configuration/Config.ts
@@ -1,5 +1,6 @@
 import {
   Difficulty,
+  Game,
   GameType,
   Gold,
   Player,
@@ -81,7 +82,7 @@ export interface Config {
     numAdjacentTilesWithEnemy: number,
   ): number;
   attackLogic(
-    gm: GameMap,
+    gm: Game,
     attackTroops: number,
     attacker: Player,
     defender: Player | TerraNullius,
diff --git a/src/core/configuration/DefaultConfig.ts b/src/core/configuration/DefaultConfig.ts
index 17e1afc64..52aeb4dbf 100644
--- a/src/core/configuration/DefaultConfig.ts
+++ b/src/core/configuration/DefaultConfig.ts
@@ -1,5 +1,6 @@
 import {
   Difficulty,
+  Game,
   GameType,
   Gold,
   Player,
@@ -66,7 +67,7 @@ export class DefaultConfig implements Config {
   }
 
   defensePostRange(): number {
-    return 40;
+    return 30;
   }
   defensePostDefenseBonus(): number {
     return 5;
@@ -205,7 +206,7 @@ export class DefaultConfig implements Config {
   }
 
   attackLogic(
-    gm: GameMap,
+    gm: Game,
     attackTroops: number,
     attacker: Player,
     defender: Player | TerraNullius,
@@ -234,9 +235,16 @@ export class DefaultConfig implements Config {
       default:
         throw new Error(`terrain type ${type} not supported`);
     }
-    // TODO
-    // mag *= tileToConquer.defenseBonus(attacker)
-    // speed *= tileToConquer.defenseBonus(attacker)
+    if (defender.isPlayer()) {
+      for (const dp of gm.nearbyDefensePosts(tileToConquer)) {
+        if (dp.owner() == defender) {
+          mag *= this.defensePostDefenseBonus();
+          speed *= this.defensePostDefenseBonus();
+          break;
+        }
+      }
+    }
+
     if (gm.hasFallout(tileToConquer)) {
       mag *= this.falloutDefenseModifier();
       speed *= this.falloutDefenseModifier();
diff --git a/src/core/execution/DefensePostExecution.ts b/src/core/execution/DefensePostExecution.ts
index ea5dcd96b..52a690a40 100644
--- a/src/core/execution/DefensePostExecution.ts
+++ b/src/core/execution/DefensePostExecution.ts
@@ -1,7 +1,6 @@
 import { consolex } from "../Consolex";
 import {
   Cell,
-  DefenseBonus,
   Execution,
   Game,
   Player,
@@ -17,12 +16,7 @@ export class DefensePostExecution implements Execution {
   private post: Unit;
   private active: boolean = true;
 
-  private defenseBonuses: DefenseBonus[] = [];
-
-  constructor(
-    private ownerId: PlayerID,
-    private tile: TileRef,
-  ) {}
+  constructor(private ownerId: PlayerID, private tile: TileRef) {}
 
   init(mg: Game, ticks: number): void {
     this.mg = mg;
@@ -38,25 +32,8 @@ export class DefensePostExecution implements Execution {
         return;
       }
       this.post = this.player.buildUnit(UnitType.DefensePost, 0, spawnTile);
-      this.mg
-        .bfs(
-          spawnTile,
-          manhattanDistFN(spawnTile, this.mg.config().defensePostRange()),
-        )
-        .forEach((t) => {
-          if (this.mg.isLake(t)) {
-            this.defenseBonuses.push(
-              this.mg.addTileDefenseBonus(
-                t,
-                this.post,
-                this.mg.config().defensePostDefenseBonus(),
-              ),
-            );
-          }
-        });
     }
     if (!this.post.isActive()) {
-      this.defenseBonuses.forEach((df) => this.mg.removeTileDefenseBonus(df));
       this.active = false;
       return;
     }
diff --git a/src/core/game/DefensePostGrid.ts b/src/core/game/DefensePostGrid.ts
new file mode 100644
index 000000000..bafffe6c8
--- /dev/null
+++ b/src/core/game/DefensePostGrid.ts
@@ -0,0 +1,89 @@
+import { Unit } from "./Game";
+import { GameMap, TileRef } from "./GameMap";
+import { UnitView } from "./GameView";
+
+export class DefenseGrid {
+  private grid: Set<Unit | UnitView>[][];
+  private readonly cellSize = 100;
+
+  constructor(private gm: GameMap, private searchRange: number) {
+    this.grid = Array(Math.ceil(gm.height() / this.cellSize))
+      .fill(null)
+      .map(() =>
+        Array(Math.ceil(gm.width() / this.cellSize))
+          .fill(null)
+          .map(() => new Set<Unit | UnitView>())
+      );
+  }
+
+  // Get grid coordinates from pixel coordinates
+  private getGridCoords(x: number, y: number): [number, number] {
+    return [Math.floor(x / this.cellSize), Math.floor(y / this.cellSize)];
+  }
+
+  // Add a defense unit to the grid
+  addDefense(unit: Unit | UnitView) {
+    const tile = unit.tile();
+    const [gridX, gridY] = this.getGridCoords(this.gm.x(tile), this.gm.y(tile));
+
+    if (this.isValidCell(gridX, gridY)) {
+      this.grid[gridY][gridX].add(unit);
+    }
+  }
+
+  // Remove a defense unit from the grid
+  removeDefense(unit: Unit | UnitView) {
+    const tile = unit.tile();
+    const [gridX, gridY] = this.getGridCoords(this.gm.x(tile), this.gm.y(tile));
+
+    if (this.isValidCell(gridX, gridY)) {
+      this.grid[gridY][gridX].delete(unit);
+    }
+  }
+
+  private isValidCell(gridX: number, gridY: number): boolean {
+    return (
+      gridX >= 0 &&
+      gridX < this.grid[0].length &&
+      gridY >= 0 &&
+      gridY < this.grid.length
+    );
+  }
+
+  // Get all defense units within range of a point
+  // Returns [unit, distanceSquared] pairs for efficient filtering
+  nearbyDefenses(tile: TileRef): Array<Unit | UnitView> {
+    const x = this.gm.x(tile);
+    const y = this.gm.y(tile);
+    const [gridX, gridY] = this.getGridCoords(x, y);
+    const cellsToCheck = Math.ceil(this.searchRange / this.cellSize);
+    const nearby: Array<Unit | UnitView> = [];
+
+    // Pre-calculate range bounds for efficiency
+    const startGridX = Math.max(0, gridX - cellsToCheck);
+    const endGridX = Math.min(this.grid[0].length - 1, gridX + cellsToCheck);
+    const startGridY = Math.max(0, gridY - cellsToCheck);
+    const endGridY = Math.min(this.grid.length - 1, gridY + cellsToCheck);
+
+    // Squared range for faster comparison (avoid sqrt)
+    const rangeSquared = this.searchRange * this.searchRange;
+
+    for (let cy = startGridY; cy <= endGridY; cy++) {
+      for (let cx = startGridX; cx <= endGridX; cx++) {
+        for (const unit of this.grid[cy][cx]) {
+          const tileX = this.gm.x(unit.tile());
+          const tileY = this.gm.y(unit.tile());
+          const dx = tileX - x;
+          const dy = tileY - y;
+          const distSquared = dx * dx + dy * dy;
+
+          if (distSquared <= rangeSquared) {
+            nearby.push(unit);
+          }
+        }
+      }
+    }
+
+    return nearby;
+  }
+}
diff --git a/src/core/game/Game.ts b/src/core/game/Game.ts
index a7d243cef..d12497f74 100644
--- a/src/core/game/Game.ts
+++ b/src/core/game/Game.ts
@@ -165,13 +165,6 @@ export class PlayerInfo {
   ) {}
 }
 
-export interface DefenseBonus {
-  // Unit providing the defense bonus
-  unit: Unit;
-  amount: number;
-  tile: TileRef;
-}
-
 export interface Unit {
   // Properties
   type(): UnitType;
@@ -323,8 +316,7 @@ export interface Game extends GameMap {
   // Units
   units(...types: UnitType[]): Unit[];
   unitInfo(type: UnitType): UnitInfo;
-  addTileDefenseBonus(tile: TileRef, unit: Unit, amount: number): DefenseBonus;
-  removeTileDefenseBonus(bonus: DefenseBonus): void;
+  nearbyDefensePosts(tile: TileRef): Unit[];
 
   // Events & Messages
   executions(): Execution[];
diff --git a/src/core/game/GameImpl.ts b/src/core/game/GameImpl.ts
index c0b89002f..ea6114a11 100644
--- a/src/core/game/GameImpl.ts
+++ b/src/core/game/GameImpl.ts
@@ -13,7 +13,6 @@ import {
   Nation,
   UnitType,
   UnitInfo,
-  DefenseBonus,
   AllPlayers,
   GameUpdates,
   TerrainType,
@@ -31,6 +30,7 @@ import { MessageType } from "./Game";
 import { UnitImpl } from "./UnitImpl";
 import { consolex } from "../Consolex";
 import { GameMap, GameMapImpl, TileRef, TileUpdate } from "./GameMap";
+import { DefenseGrid } from "./DefensePostGrid";
 
 export function createGame(
   gameMap: GameMap,
@@ -65,6 +65,7 @@ export class GameImpl implements Game {
   private _nextUnitID = 1;
 
   private updates: GameUpdates = createGameUpdatesMap();
+  private defenseGrid: DefenseGrid;
 
   constructor(
     private _map: GameMap,
@@ -83,6 +84,10 @@ export class GameImpl implements Game {
           n.strength
         )
     );
+    this.defenseGrid = new DefenseGrid(
+      this._map,
+      this._config.defensePostRange()
+    );
   }
 
   owner(ref: TileRef): Player | TerraNullius {
@@ -125,21 +130,6 @@ export class GameImpl implements Game {
     });
   }
 
-  addTileDefenseBonus(tile: TileRef, unit: Unit, amount: number): DefenseBonus {
-    // TODO!!
-    const df = { unit: unit, tile: tile, amount: amount };
-    // (tile as TileImpl)._defenseBonuses.push(df)
-    // this.addUpdate((tile as TileImpl).toUpdate())
-    return df;
-  }
-
-  removeTileDefenseBonus(bonus: DefenseBonus): void {
-    // TODO!!
-    // const t = bonus.tile as TileImpl
-    // t._defenseBonuses = t._defenseBonuses.filter(db => db != bonus)
-    // this.addUpdate(t.toUpdate())
-  }
-
   units(...types: UnitType[]): UnitImpl[] {
     return Array.from(this._players.values()).flatMap((p) => p.units(...types));
   }
@@ -425,7 +415,6 @@ export class GameImpl implements Game {
       } else {
         (this.owner(t) as PlayerImpl)._borderTiles.delete(t);
       }
-      // this.updates.push(t.toUpdate())
     }
   }
 
@@ -531,6 +520,17 @@ export class GameImpl implements Game {
     });
   }
 
+  addDefensePost(dp: Unit) {
+    this.defenseGrid.addDefense(dp);
+  }
+  removeDefensePost(dp: Unit) {
+    this.defenseGrid.removeDefense(dp);
+  }
+
+  nearbyDefensePosts(tile: TileRef): Unit[] {
+    return this.defenseGrid.nearbyDefenses(tile) as Unit[];
+  }
+
   ref(x: number, y: number): TileRef {
     return this._map.ref(x, y);
   }
diff --git a/src/core/game/GameUpdates.ts b/src/core/game/GameUpdates.ts
index 840cbd03a..ad1c3eaf8 100644
--- a/src/core/game/GameUpdates.ts
+++ b/src/core/game/GameUpdates.ts
@@ -62,6 +62,7 @@ export interface UnitUpdate {
   troops: number;
   id: number;
   ownerID: number;
+  // TODO: make these tilerefs
   pos: MapPos;
   lastPos: MapPos;
   isActive: boolean;
diff --git a/src/core/game/GameView.ts b/src/core/game/GameView.ts
index 1af2860b6..7b87026f5 100644
--- a/src/core/game/GameView.ts
+++ b/src/core/game/GameView.ts
@@ -13,19 +13,12 @@ import { NameViewData } from "./Game";
 import { GameUpdateType } from "./GameUpdates";
 import { Config } from "../configuration/Config";
 import {
-  Alliance,
-  AllianceRequest,
-  AllPlayers,
   Cell,
-  DefenseBonus,
   EmojiMessage,
-  Game,
   Gold,
-  Nation,
   PlayerID,
   PlayerInfo,
   PlayerType,
-  Relation,
   TerrainType,
   TerraNullius,
   Tick,
@@ -37,6 +30,7 @@ import { TerraNulliusImpl } from "./TerraNulliusImpl";
 import { WorkerClient } from "../worker/WorkerClient";
 import { GameMap, GameMapImpl, TileRef, TileUpdate } from "./GameMap";
 import { GameUpdateViewData } from "./GameUpdates";
+import { DefenseGrid } from "./DefensePostGrid";
 
 export class UnitView {
   public _wasUpdated = true;
@@ -204,6 +198,8 @@ export class GameView implements GameMap {
 
   private _myPlayer: PlayerView | null = null;
 
+  private defensePostGrid: DefenseGrid;
+
   constructor(
     public worker: WorkerClient,
     private _config: Config,
@@ -217,6 +213,7 @@ export class GameView implements GameMap {
       updates: null,
       playerNameViewData: {},
     };
+    this.defensePostGrid = new DefenseGrid(_map, _config.defensePostRange());
   }
 
   public updatesSinceLastTick(): GameUpdates {
@@ -247,11 +244,21 @@ export class GameView implements GameMap {
       unit._wasUpdated = false;
       unit.lastPos = unit.lastPos.slice(-1);
     }
-    gu.updates[GameUpdateType.Unit].forEach((unit) => {
-      if (this._units.has(unit.id)) {
-        this._units.get(unit.id).update(unit);
+    gu.updates[GameUpdateType.Unit].forEach((update) => {
+      let unit: UnitView = null;
+      if (this._units.has(update.id)) {
+        unit = this._units.get(update.id);
+        unit.update(update);
       } else {
-        this._units.set(unit.id, new UnitView(this, unit));
+        unit = new UnitView(this, update);
+        this._units.set(update.id, unit);
+      }
+      if (update.unitType == UnitType.DefensePost) {
+        if (update.isActive) {
+          this.defensePostGrid.addDefense(unit);
+        } else {
+          this.defensePostGrid.removeDefense(unit);
+        }
       }
     });
   }
@@ -260,6 +267,10 @@ export class GameView implements GameMap {
     return this.updatedTiles;
   }
 
+  nearbyDefenses(tile: TileRef): UnitView[] {
+    return this.defensePostGrid.nearbyDefenses(tile) as UnitView[];
+  }
+
   myClientID(): ClientID {
     return this._myClientID;
   }
diff --git a/src/core/game/PlayerImpl.ts b/src/core/game/PlayerImpl.ts
index 9c0ce7904..97dac7417 100644
--- a/src/core/game/PlayerImpl.ts
+++ b/src/core/game/PlayerImpl.ts
@@ -556,6 +556,9 @@ export class PlayerImpl implements Player {
     this.removeGold(cost);
     this.removeTroops(troops);
     this.mg.addUpdate(b.toUpdate());
+    if (type == UnitType.DefensePost) {
+      this.mg.addDefensePost(b);
+    }
     return b;
   }
 
diff --git a/src/core/game/UnitImpl.ts b/src/core/game/UnitImpl.ts
index f31089e8b..4429714c9 100644
--- a/src/core/game/UnitImpl.ts
+++ b/src/core/game/UnitImpl.ts
@@ -101,6 +101,9 @@ export class UnitImpl implements Unit {
     this._owner._units = this._owner._units.filter((b) => b != this);
     this._active = false;
     this.mg.addUpdate(this.toUpdate());
+    if (this.type() == UnitType.DefensePost) {
+      this.mg.removeDefensePost(this);
+    }
     if (displayMessage) {
       this.mg.displayMessage(
         `Your ${this.type()} was destroyed`,