diff --git a/resources/lang/en.json b/resources/lang/en.json
index 5947ad4e2..be506ca04 100644
--- a/resources/lang/en.json
+++ b/resources/lang/en.json
@@ -146,7 +146,7 @@
     "build_icon": "Icon",
     "build_desc": "Description",
     "build_city": "City",
-    "build_city_desc": "Increases your max population. Useful when you can't expand your territory or you're about to hit your population limit.",
+    "build_city_desc": "A city that can be expanded by double-clicking (costs gold and troops). Its population and gold generation increase with area and density. Denser areas are harder to capture.",
     "build_factory": "Factory",
     "build_factory_desc": "Automatically builds railroads to nearby cities, ports and other factories, and can also link up with friendly neighbors. Trains spawn regularly and give you a fixed amount of gold for each building they visit along the route, with extra gold for visiting your neighbors' buildings.",
     "build_defense": "Defense Post",
@@ -752,6 +752,8 @@
     "received_gold_from_player": "Received {gold} gold from {name}",
     "unit_captured_by_enemy": "Your {unit} was captured by {name}",
     "captured_enemy_unit": "Captured {unit} from {name}",
+    "unit_expanded": "Expanded {unit} to level {level}",
+    "double_click_expand_hint": "Double-click to expand (costs gold & troops)",
     "unit_destroyed": "Your {unit} was destroyed",
     "no_boats_available": "No boats available, max {max}"
   },
@@ -773,6 +775,8 @@
     "betrayals": "Betrayals",
     "traitor": "Traitor",
     "trading": "Trading",
+    "population": "Population",
+    "density": "Density",
     "active": "Active",
     "stopped": "Stopped",
     "alliance_time_remaining": "Alliance Expires In",
diff --git a/src/client/InputHandler.ts b/src/client/InputHandler.ts
index 45d1188a3..775abf0bc 100644
--- a/src/client/InputHandler.ts
+++ b/src/client/InputHandler.ts
@@ -12,6 +12,13 @@ export class MouseUpEvent implements GameEvent {
   ) {}
 }
 
+export class DoubleClickEvent implements GameEvent {
+  constructor(
+    public readonly x: number,
+    public readonly y: number,
+  ) {}
+}
+
 export class MouseOverEvent implements GameEvent {
   constructor(
     public readonly x: number,
@@ -75,6 +82,10 @@ export class AlternateViewEvent implements GameEvent {
   constructor(public readonly alternateView: boolean) {}
 }
 
+export class CityUpdateEvent implements GameEvent {
+  constructor(public readonly city: UnitView) {}
+}
+
 export class CloseViewEvent implements GameEvent {}
 
 export class RefreshGraphicsEvent implements GameEvent {}
@@ -148,6 +159,9 @@ export class InputHandler {
   private lastPinchDistance: number = 0;
 
   private pointerDown: boolean = false;
+  private lastClickTime: number = 0;
+  private lastClickX: number = 0;
+  private lastClickY: number = 0;
 
   private alternateView = false;
 
@@ -517,6 +531,19 @@ export class InputHandler {
         return;
       }
 
+      const now = Date.now();
+      const clickDist =
+        Math.abs(event.x - this.lastClickX) +
+        Math.abs(event.y - this.lastClickY);
+      if (now - this.lastClickTime < 300 && clickDist < 20) {
+        this.eventBus.emit(new DoubleClickEvent(event.x, event.y));
+        this.lastClickTime = 0; // reset to prevent triple-click double events
+        return;
+      }
+      this.lastClickTime = now;
+      this.lastClickX = event.x;
+      this.lastClickY = event.y;
+
       if (!this.userSettings.leftClickOpensMenu() || event.shiftKey) {
         this.eventBus.emit(new MouseUpEvent(event.x, event.y));
       } else {
diff --git a/src/client/graphics/GameRenderer.ts b/src/client/graphics/GameRenderer.ts
index 5f4ef4a37..e9f96df79 100644
--- a/src/client/graphics/GameRenderer.ts
+++ b/src/client/graphics/GameRenderer.ts
@@ -428,7 +428,11 @@ export class GameRenderer {
       );
 
       const layerStart = FrameProfiler.start();
-      layer.renderLayer?.(this.context);
+      try {
+        layer.renderLayer?.(this.context);
+      } catch (e) {
+        console.error(`Error rendering layer ${layer.constructor.name}:`, e);
+      }
       FrameProfiler.end(layer.constructor?.name ?? "UnknownLayer", layerStart);
     }
     handleTransformState(false, isTransformActive); // Ensure context is clean after rendering
diff --git a/src/client/graphics/layers/PlayerInfoOverlay.ts b/src/client/graphics/layers/PlayerInfoOverlay.ts
index b62a22d3e..4e0470045 100644
--- a/src/client/graphics/layers/PlayerInfoOverlay.ts
+++ b/src/client/graphics/layers/PlayerInfoOverlay.ts
@@ -141,6 +141,13 @@ export class PlayerInfoOverlay extends LitElement implements Layer {
       this.player.profile().then((p) => {
         this.playerProfile = p;
       });
+
+      // Also check if we are hovering a city within this player's territory
+      const urbanization = this.game.getUrbanization(tile);
+      if (urbanization.unit) {
+        this.unit = urbanization.unit;
+      }
+
       this.setVisible(true);
     } else if (!this.game.isLand(tile)) {
       const units = this.game
@@ -172,6 +179,33 @@ export class PlayerInfoOverlay extends LitElement implements Layer {
     this.requestUpdate();
   }
 
+  private getUnitTypeKey(type: UnitType): string {
+    switch (type) {
+      case UnitType.City:
+        return "city";
+      case UnitType.DefensePost:
+        return "defense_post";
+      case UnitType.Port:
+        return "port";
+      case UnitType.Warship:
+        return "warship";
+      case UnitType.MissileSilo:
+        return "missile_silo";
+      case UnitType.SAMLauncher:
+        return "sam_launcher";
+      case UnitType.AtomBomb:
+        return "atom_bomb";
+      case UnitType.HydrogenBomb:
+        return "hydrogen_bomb";
+      case UnitType.MIRV:
+        return "mirv";
+      case UnitType.Factory:
+        return "factory";
+      default:
+        return type.toLowerCase().replace(/ /g, "_");
+    }
+  }
+
   private getPlayerNameColor(
     player: PlayerView,
     myPlayer: PlayerView | null | undefined,
@@ -456,15 +490,44 @@ export class PlayerInfoOverlay extends LitElement implements Layer {
         this.game.myPlayer()?.isFriendly(unit.owner())) ??
       false;
 
+    const isCity = unit.type() === UnitType.City;
+    const area = Math.PI * Math.pow(unit.areaRadius(), 2);
+    const population = Math.floor(area * 1500 * (1 + unit.density()));
+
     return html`
-      <div class="p-2">
+      <div class="p-2 border-t border-gray-700/50">
         <div class="font-bold mb-1 ${isAlly ? "text-green-500" : "text-white"}">
           ${unit.owner().name()}
         </div>
         <div class="mt-1">
-          <div class="text-sm opacity-80">${unit.type()}</div>
+          <div class="text-sm opacity-80">
+            ${translateText("unit_type." + this.getUnitTypeKey(unit.type()))}
+          </div>
+          ${isCity
+            ? html`
+                <div class="text-xs text-blue-300">
+                  ${translateText("player_panel.population")}:
+                  ${renderNumber(population)}
+                </div>
+                <div class="text-xs text-blue-200">
+                  ${translateText("player_panel.density")}:
+                  ${Math.round(unit.density() * 100)}%
+                </div>
+                ${unit.owner().isMe() && !unit.isUnderConstruction()
+                  ? html`<div class="text-[10px] text-gray-400 mt-1 italic">
+                      ${translateText(
+                        "events_display.double_click_expand_hint",
+                      )}
+                    </div>`
+                  : ""}
+              `
+            : ""}
           ${unit.hasHealth()
-            ? html` <div class="text-sm">Health: ${unit.health()}</div> `
+            ? html`
+                <div class="text-sm">
+                  ${translateText("player_stats_table.hits")}: ${unit.health()}
+                </div>
+              `
             : ""}
           ${unit.type() === UnitType.TransportShip
             ? html`
diff --git a/src/client/graphics/layers/StructureDrawingUtils.ts b/src/client/graphics/layers/StructureDrawingUtils.ts
index 9fe6a940b..452f496a7 100644
--- a/src/client/graphics/layers/StructureDrawingUtils.ts
+++ b/src/client/graphics/layers/StructureDrawingUtils.ts
@@ -88,6 +88,8 @@ export class SpriteFactory {
     image.onload = () => {
       unitInfo.image = image;
       this.invalidateTextureCache(unitType);
+      // We don't have direct access to layers to call redraw,
+      // but the next tick will pick up the new texture.
     };
     image.onerror = () => {
       console.error(
@@ -411,7 +413,7 @@ export class SpriteFactory {
 
     const structureInfo = this.structuresInfos.get(structureType);
 
-    if (structureInfo?.image && renderIcon) {
+    if (structureInfo?.image && structureInfo.image.width > 0 && renderIcon) {
       const SHAPE_OFFSETS = {
         triangle: [6, 11],
         square: [5, 5],
diff --git a/src/client/graphics/layers/StructureIconsLayer.ts b/src/client/graphics/layers/StructureIconsLayer.ts
index 72d7ef7f9..c8b752f45 100644
--- a/src/client/graphics/layers/StructureIconsLayer.ts
+++ b/src/client/graphics/layers/StructureIconsLayer.ts
@@ -16,6 +16,8 @@ import { TileRef } from "../../../core/game/GameMap";
 import { GameUpdateType } from "../../../core/game/GameUpdates";
 import { GameView, UnitView } from "../../../core/game/GameView";
 import {
+  CityUpdateEvent,
+  DoubleClickEvent,
   GhostStructureChangedEvent,
   MouseMoveEvent,
   MouseUpEvent,
@@ -172,6 +174,7 @@ export class StructureIconsLayer implements Layer {
     this.eventBus.on(MouseMoveEvent, (e) => this.moveGhost(e));
 
     this.eventBus.on(MouseUpEvent, (e) => this.createStructure(e));
+    this.eventBus.on(DoubleClickEvent, (e) => this.onDoubleClick(e));
 
     window.addEventListener("resize", () => this.resizeCanvas());
     await this.setupRenderer();
@@ -419,6 +422,33 @@ export class StructureIconsLayer implements Layer {
     this.removeGhostStructure();
   }
 
+  private onDoubleClick(e: DoubleClickEvent) {
+    const rect = this.transformHandler.boundingRect();
+    if (!rect) return;
+    const x = e.x - rect.left;
+    const y = e.y - rect.top;
+    const tile = this.transformHandler.screenToWorldCoordinates(x, y);
+    if (!this.game.isValidCoord(tile.x, tile.y)) return;
+    const tileRef = this.game.ref(tile.x, tile.y);
+
+    // Find city at or near this tile
+    const radius = 10; // tolerance for clicking
+    const nearby = this.game.nearbyUnits(tileRef, radius, UnitType.City);
+    if (nearby.length > 0) {
+      // Sort by distance to find the closest city
+      nearby.sort((a, b) => a.distSquared - b.distSquared);
+      const city = nearby[0].unit;
+      if (
+        city.owner() === this.game.myPlayer() &&
+        !city.isUnderConstruction()
+      ) {
+        this.eventBus.emit(
+          new SendUpgradeStructureIntentEvent(city.id(), UnitType.City),
+        );
+      }
+    }
+  }
+
   private moveGhost(e: MouseMoveEvent) {
     this.mousePos.x = e.x;
     this.mousePos.y = e.y;
@@ -558,13 +588,26 @@ export class StructureIconsLayer implements Layer {
     if (this.seenUnits.has(unitView)) {
       const render = this.findRenderByUnit(unitView);
       if (render) {
-        this.checkForConstructionState(render, unitView);
-        this.checkForDeletionState(render, unitView);
-        this.checkForOwnershipChange(render, unitView);
-        this.checkForLevelChange(render, unitView);
+        let changed = false;
+        if (this.checkForConstructionState(render, unitView)) changed = true;
+        if (this.checkForDeletionState(render, unitView)) changed = true;
+        if (this.checkForOwnershipChange(render, unitView)) changed = true;
+        if (this.checkForLevelChange(render, unitView)) changed = true;
+        if (this.checkForDensityChange(render, unitView)) changed = true;
+
+        if (changed) {
+          this.computeNewLocation(render);
+          if (unitView.type() === UnitType.City) {
+            this.eventBus.emit(new CityUpdateEvent(unitView));
+          }
+        }
       }
     } else if (this.structures.has(unitView.type())) {
+      const city = unitView.type() === UnitType.City;
       this.addNewStructure(unitView);
+      if (city) {
+        this.eventBus.emit(new CityUpdateEvent(unitView));
+      }
     }
   }
 
@@ -603,20 +646,25 @@ export class StructureIconsLayer implements Layer {
     }
   }
 
-  private checkForDeletionState(render: StructureRenderInfo, unit: UnitView) {
+  private checkForDeletionState(
+    render: StructureRenderInfo,
+    unit: UnitView,
+  ): boolean {
     if (unit.markedForDeletion() !== false) {
       render.iconContainer?.destroy();
       render.dotContainer?.destroy();
       render.iconContainer = this.createIconSprite(unit);
       render.dotContainer = this.createDotSprite(unit);
       this.modifyVisibility(render);
+      return true;
     }
+    return false;
   }
 
   private checkForConstructionState(
     render: StructureRenderInfo,
     unit: UnitView,
-  ) {
+  ): boolean {
     if (render.underConstruction && !unit.isUnderConstruction()) {
       render.underConstruction = false;
       render.iconContainer?.destroy();
@@ -624,10 +672,15 @@ export class StructureIconsLayer implements Layer {
       render.iconContainer = this.createIconSprite(unit);
       render.dotContainer = this.createDotSprite(unit);
       this.modifyVisibility(render);
+      return true;
     }
+    return false;
   }
 
-  private checkForOwnershipChange(render: StructureRenderInfo, unit: UnitView) {
+  private checkForOwnershipChange(
+    render: StructureRenderInfo,
+    unit: UnitView,
+  ): boolean {
     if (render.owner !== unit.owner().id()) {
       render.owner = unit.owner().id();
       render.iconContainer?.destroy();
@@ -635,10 +688,15 @@ export class StructureIconsLayer implements Layer {
       render.iconContainer = this.createIconSprite(unit);
       render.dotContainer = this.createDotSprite(unit);
       this.modifyVisibility(render);
+      return true;
     }
+    return false;
   }
 
-  private checkForLevelChange(render: StructureRenderInfo, unit: UnitView) {
+  private checkForLevelChange(
+    render: StructureRenderInfo,
+    unit: UnitView,
+  ): boolean {
     if (render.level !== unit.level()) {
       render.level = unit.level();
       render.iconContainer?.destroy();
@@ -648,7 +706,32 @@ export class StructureIconsLayer implements Layer {
       render.levelContainer = this.createLevelSprite(unit);
       render.dotContainer = this.createDotSprite(unit);
       this.modifyVisibility(render);
+      return true;
     }
+    return false;
+  }
+
+  private lastDensity = new Map<number, number>();
+
+  private checkForDensityChange(
+    render: StructureRenderInfo,
+    unit: UnitView,
+  ): boolean {
+    if (unit.type() !== UnitType.City || !unit.isActive() || !unit.owner())
+      return false;
+    const currentDensity = Math.round(unit.density() * 10) / 10;
+    const prevDensity = this.lastDensity.get(unit.id()) ?? 0;
+
+    if (currentDensity !== prevDensity) {
+      this.lastDensity.set(unit.id(), currentDensity);
+      render.iconContainer?.destroy();
+      render.dotContainer?.destroy();
+      render.iconContainer = this.createIconSprite(unit);
+      render.dotContainer = this.createDotSprite(unit);
+      this.modifyVisibility(render);
+      return true;
+    }
+    return false;
   }
 
   private computeNewLocation(render: StructureRenderInfo) {
@@ -753,5 +836,6 @@ export class StructureIconsLayer implements Layer {
     render.dotContainer?.destroy();
     this.renders = this.renders.filter((r) => r.unit !== render.unit);
     this.seenUnits.delete(render.unit);
+    this.lastDensity.delete(render.unit.id());
   }
 }
diff --git a/src/client/graphics/layers/TerritoryLayer.ts b/src/client/graphics/layers/TerritoryLayer.ts
index 08d3f5a9c..bcefbe8e7 100644
--- a/src/client/graphics/layers/TerritoryLayer.ts
+++ b/src/client/graphics/layers/TerritoryLayer.ts
@@ -16,6 +16,7 @@ import { UserSettings } from "../../../core/game/UserSettings";
 import { PseudoRandom } from "../../../core/PseudoRandom";
 import {
   AlternateViewEvent,
+  CityUpdateEvent,
   DragEvent,
   MouseOverEvent,
 } from "../../InputHandler";
@@ -330,6 +331,9 @@ export class TerritoryLayer implements Layer {
       // TODO: consider re-enabling this on mobile or low end devices for smoother dragging.
       // this.lastDragTime = Date.now();
     });
+    this.eventBus.on(CityUpdateEvent, (e) => {
+      this.enqueueCityArea(e.city);
+    });
     this.redraw();
   }
 
@@ -586,7 +590,29 @@ export class TerritoryLayer implements Layer {
       // Alternative view only shows borders.
       this.clearAlternativeTile(tile);
 
-      this.paintTile(this.imageData, tile, owner.territoryColor(tile), 150);
+      if (!owner) {
+        this.clearTile(tile);
+        return;
+      }
+
+      const urbanization = this.game.getUrbanization(tile);
+      let color = owner.territoryColor(tile);
+      let alpha = 150;
+
+      if (urbanization.density > 0.05) {
+        // Alpha increases with density, making urban centers feel more "heavy" and established
+        alpha = 160 + Math.min(85, urbanization.density * 70);
+
+        // Lightness Gradient: Rural (Low density) is lighter, Urban (High density) is darker/richer.
+        // We interpolate between a "rural" shade and an "urban" shade of the player's color.
+        const ruralShade = color.lighten(0.15).desaturate(0.05);
+        const urbanShade = color.darken(0.15).saturate(0.2);
+
+        const mixRatio = Math.min(1, urbanization.density);
+        color = ruralShade.mix(urbanShade, mixRatio);
+      }
+
+      this.paintTile(this.imageData, tile, color, alpha);
     }
   }
 
@@ -638,6 +664,21 @@ export class TerritoryLayer implements Layer {
     });
   }
 
+  enqueueCityArea(city: UnitView) {
+    const radius = Math.ceil(city.areaRadius() * 1.4 + 2);
+    const tile = city.tile();
+    const cx = this.game.x(tile);
+    const cy = this.game.y(tile);
+
+    for (let x = cx - radius; x <= cx + radius; x++) {
+      for (let y = cy - radius; y <= cy + radius; y++) {
+        if (this.game.isValidCoord(x, y)) {
+          this.enqueueTile(this.game.ref(x, y));
+        }
+      }
+    }
+  }
+
   async enqueuePlayerBorder(player: PlayerView) {
     const playerBorderTiles = await player.borderTiles();
     playerBorderTiles.borderTiles.forEach((tile: TileRef) => {
diff --git a/src/core/configuration/Colors.ts b/src/core/configuration/Colors.ts
index 37caba0f3..62cce3953 100644
--- a/src/core/configuration/Colors.ts
+++ b/src/core/configuration/Colors.ts
@@ -1,9 +1,13 @@
 import { colord, Colord, extend } from "colord";
+import a11yPlugin from "colord/plugins/a11y";
 import labPlugin from "colord/plugins/lab";
 import lchPlugin from "colord/plugins/lch";
+import mixPlugin from "colord/plugins/mix";
 
 extend([lchPlugin]);
 extend([labPlugin]);
+extend([mixPlugin]);
+extend([a11yPlugin]);
 
 export const red = colord("rgb(235,51,51)");
 export const blue = colord("rgb(41,98,255)");
diff --git a/src/core/configuration/DefaultConfig.ts b/src/core/configuration/DefaultConfig.ts
index 5a672f296..8800b0cf9 100644
--- a/src/core/configuration/DefaultConfig.ts
+++ b/src/core/configuration/DefaultConfig.ts
@@ -590,6 +590,27 @@ export class DefaultConfig implements Config {
         throw new Error(`terrain type ${type} not supported`);
     }
     if (defender.isPlayer()) {
+      // City defense bonus based on density and area
+      const maxCitySearchRadius = 50;
+      const nearbyCities = gm.nearbyUnits(
+        tileToConquer,
+        maxCitySearchRadius,
+        UnitType.City,
+        ({ unit: u }) => u.owner() === defender && !u.isUnderConstruction(),
+      );
+      for (const { unit: city, distSquared } of nearbyCities) {
+        const areaRadius = city.areaRadius() || 0;
+        if (areaRadius > 0 && Math.sqrt(distSquared) <= areaRadius) {
+          // Denser cities are harder to capture
+          // Density bonus scales from 1.5x up to ~3x or more based on age and area
+          const density = city.density() || 0;
+          const densityBonus = 1.5 + density * 2;
+          mag *= densityBonus;
+          speed *= 0.7; // Moving through a city is slower
+          break;
+        }
+      }
+
       for (const dp of gm.nearbyUnits(
         tileToConquer,
         gm.config().defensePostRange(),
@@ -657,7 +678,8 @@ export class DefaultConfig implements Config {
           largeDefenderAttackDebuff *
           largeAttackBonus *
           (defender.isTraitor() ? this.traitorDefenseDebuff() : 1),
-        defenderTroopLoss: defender.troops() / defender.numTilesOwned(),
+        defenderTroopLoss:
+          defender.troops() / Math.max(1, defender.numTilesOwned()),
         tilesPerTickUsed:
           within(defender.troops() / (5 * attackTroops), 0.2, 1.5) *
           speed *
@@ -752,27 +774,31 @@ export class DefaultConfig implements Config {
         : 2 * (Math.pow(player.numTilesOwned(), 0.6) * 1000 + 50000) +
           player
             .units(UnitType.City)
-            .map((city) => city.level())
-            .reduce((a, b) => a + b, 0) *
-            this.cityTroopIncrease();
+            .filter((city) => !city.isUnderConstruction())
+            .reduce((acc, city) => {
+              const area = Math.PI * Math.pow(city.areaRadius() || 1, 2);
+              const density = city.density() || 0;
+              const population = area * 1500 * (1 + density);
+              return acc + (isNaN(population) ? 0 : population);
+            }, 0);
 
     if (player.type() === PlayerType.Bot) {
-      return maxTroops / 3;
+      return Math.max(1, maxTroops / 3);
     }
 
     if (player.type() === PlayerType.Human) {
-      return maxTroops;
+      return Math.max(1, maxTroops);
     }
 
     switch (this._gameConfig.difficulty) {
       case Difficulty.Easy:
-        return maxTroops * 0.5;
+        return Math.max(1, maxTroops * 0.5);
       case Difficulty.Medium:
-        return maxTroops * 0.75;
+        return Math.max(1, maxTroops * 0.75);
       case Difficulty.Hard:
-        return maxTroops * 1; // Like humans
+        return Math.max(1, maxTroops * 1); // Like humans
       case Difficulty.Impossible:
-        return maxTroops * 1.25;
+        return Math.max(1, maxTroops * 1.25);
       default:
         assertNever(this._gameConfig.difficulty);
     }
@@ -820,7 +846,22 @@ export class DefaultConfig implements Config {
     } else {
       baseRate = 100n;
     }
-    return BigInt(Math.floor(Number(baseRate) * multiplier));
+
+    // Cities generate extra gold based on area and density
+    let cityIncome = 0;
+    for (const city of player.units(UnitType.City)) {
+      if (!city.isUnderConstruction()) {
+        const area = Math.PI * Math.pow(city.areaRadius() || 1, 2);
+        const density = city.density() || 0;
+        cityIncome += area * 0.1 * (1 + density);
+      }
+    }
+
+    if (isNaN(cityIncome)) {
+      cityIncome = 0;
+    }
+
+    return BigInt(Math.floor((Number(baseRate) + cityIncome) * multiplier));
   }
 
   nukeMagnitudes(unitType: UnitType): NukeMagnitude {
diff --git a/src/core/execution/CityExecution.ts b/src/core/execution/CityExecution.ts
index fd31c940e..d42143cdb 100644
--- a/src/core/execution/CityExecution.ts
+++ b/src/core/execution/CityExecution.ts
@@ -21,6 +21,7 @@ export class CityExecution implements Execution {
       this.active = false;
       return;
     }
+    this.city.incrementAge();
   }
 
   isActive(): boolean {
diff --git a/src/core/execution/nation/NationStructureBehavior.ts b/src/core/execution/nation/NationStructureBehavior.ts
index c6e937985..7297b8b2a 100644
--- a/src/core/execution/nation/NationStructureBehavior.ts
+++ b/src/core/execution/nation/NationStructureBehavior.ts
@@ -401,6 +401,15 @@ export class NationStructureBehavior {
     for (const structure of upgradable) {
       let score = 0;
 
+      // Cities are more valuable now, so prioritize them
+      if (structure.type() === UnitType.City) {
+        score += 20;
+        // Higher priority for smaller cities to help them reach density quicker
+        if (structure.level() < 5) {
+          score += 15;
+        }
+      }
+
       // Check if protected by any SAM, using per-SAM level-based range
       for (const sam of samLaunchers) {
         const samRange = this.game.config().samRange(sam.level());
diff --git a/src/core/game/Game.ts b/src/core/game/Game.ts
index 3897da573..f9a0a97e5 100644
--- a/src/core/game/Game.ts
+++ b/src/core/game/Game.ts
@@ -564,6 +564,12 @@ export interface Unit {
   // Warships
   setPatrolTile(tile: TileRef): void;
   patrolTile(): TileRef | undefined;
+
+  // Cities
+  areaRadius(): number;
+  age(): number;
+  density(): number;
+  incrementAge(): void;
 }
 
 export interface TerraNullius {
@@ -792,7 +798,7 @@ export interface Game extends GameMap {
   nearbyUnits(
     tile: TileRef,
     searchRange: number,
-    types: UnitType | UnitType[],
+    types: UnitType | readonly UnitType[],
     predicate?: UnitPredicate,
     includeUnderConstruction?: boolean,
   ): Array<{ unit: Unit; distSquared: number }>;
diff --git a/src/core/game/GameImpl.ts b/src/core/game/GameImpl.ts
index 45d77878f..417128f57 100644
--- a/src/core/game/GameImpl.ts
+++ b/src/core/game/GameImpl.ts
@@ -893,7 +893,7 @@ export class GameImpl implements Game {
   nearbyUnits(
     tile: TileRef,
     searchRange: number,
-    types: UnitType | UnitType[],
+    types: UnitType | readonly UnitType[],
     predicate?: UnitPredicate,
     includeUnderConstruction?: boolean,
   ): Array<{ unit: Unit; distSquared: number }> {
diff --git a/src/core/game/GameUpdates.ts b/src/core/game/GameUpdates.ts
index b8786e14b..3b225b3a5 100644
--- a/src/core/game/GameUpdates.ts
+++ b/src/core/game/GameUpdates.ts
@@ -140,6 +140,8 @@ export interface UnitUpdate {
   hasTrainStation: boolean;
   trainType?: TrainType; // Only for trains
   loaded?: boolean; // Only for trains
+  areaRadius?: number; // Only for cities
+  age?: number; // Only for cities
 }
 
 export interface AttackUpdate {
diff --git a/src/core/game/GameView.ts b/src/core/game/GameView.ts
index 07d2841e8..d706f70ce 100644
--- a/src/core/game/GameView.ts
+++ b/src/core/game/GameView.ts
@@ -171,6 +171,31 @@ export class UnitView {
   level(): number {
     return this.data.level;
   }
+  areaRadius(): number {
+    return this.data.areaRadius ?? 7;
+  }
+  age(): number {
+    return this.data.age ?? 0;
+  }
+
+  private _density: number | null = null;
+  private _lastDensityTick = -1;
+
+  density(): number {
+    if (this.type() !== UnitType.City) return 0;
+    const tick = this.gameView.ticks();
+    if (tick === this._lastDensityTick && this._density !== null) {
+      return this._density;
+    }
+
+    const timeConstant = 200 * this.level();
+    const maxDensity = 0.5 + (this.level() - 1) * 0.5;
+    const density = (1 - Math.exp(-this.age() / timeConstant)) * maxDensity;
+
+    this._density = Math.max(0, density) || 0;
+    this._lastDensityTick = tick;
+    return this._density;
+  }
   hasTrainStation(): boolean {
     return this.data.hasTrainStation;
   }
@@ -199,6 +224,9 @@ export class PlayerView {
   private _borderColorDefendedFriendly: { light: Colord; dark: Colord };
   private _borderColorDefendedEmbargo: { light: Colord; dark: Colord };
 
+  private _urbanizationBuildingColor: Colord;
+  private _urbanizationCoreColor: Colord;
+
   constructor(
     private game: GameView,
     public data: PlayerUpdate,
@@ -312,12 +340,23 @@ export class PlayerView {
       this._borderColorEmbargo,
     );
 
+    this._urbanizationBuildingColor = this._borderColor;
+    this._urbanizationCoreColor = this._borderColor.lighten(0.2);
+
     this.decoder =
       pattern === undefined
         ? undefined
         : new PatternDecoder(pattern, base64url.decode);
   }
 
+  urbanizationBuildingColor(): Colord {
+    return this._urbanizationBuildingColor;
+  }
+
+  urbanizationCoreColor(): Colord {
+    return this._urbanizationCoreColor;
+  }
+
   territoryColor(tile?: TileRef): Colord {
     if (tile === undefined || this.decoder === undefined) {
       return this._territoryColor;
@@ -707,14 +746,16 @@ export class GameView implements GameMap {
   nearbyUnits(
     tile: TileRef,
     searchRange: number,
-    types: UnitType | UnitType[],
+    types: UnitType | readonly UnitType[],
     predicate?: UnitPredicate,
+    includeUnderConstruction?: boolean,
   ): Array<{ unit: UnitView; distSquared: number }> {
     return this.unitGrid.nearbyUnits(
       tile,
       searchRange,
       types,
       predicate,
+      includeUnderConstruction,
     ) as Array<{
       unit: UnitView;
       distSquared: number;
@@ -954,4 +995,76 @@ export class GameView implements GameMap {
   focusedPlayer(): PlayerView | null {
     return this.myPlayer();
   }
+
+  private urbanizationCache: Map<
+    TileRef,
+    { density: number; unit?: UnitView }
+  > = new Map();
+  private lastUrbanizationUpdate = -1;
+
+  getUrbanization(tile: TileRef): { density: number; unit?: UnitView } {
+    if (!this.isLand(tile)) return { density: 0 };
+
+    if (this.ticks() !== this.lastUrbanizationUpdate) {
+      this.urbanizationCache.clear();
+      this.lastUrbanizationUpdate = this.ticks();
+    }
+
+    const cached = this.urbanizationCache.get(tile);
+    if (cached) return cached;
+
+    const nearby = this.nearbyUnits(tile, 40, UnitType.City);
+    if (nearby.length === 0) {
+      const result = { density: 0 };
+      this.urbanizationCache.set(tile, result);
+      return result;
+    }
+
+    let maxDensity = 0;
+    let maxUnit: UnitView | undefined = undefined;
+
+    const tileOwnerID = this.ownerID(tile);
+    const tx = this.x(tile);
+    const ty = this.y(tile);
+
+    for (let i = 0; i < nearby.length; i++) {
+      const { unit, distSquared } = nearby[i];
+      if (unit.isUnderConstruction()) continue;
+
+      const owner = unit.owner();
+      if (!owner || owner.smallID() !== tileOwnerID) continue;
+
+      const dist = Math.sqrt(distSquared);
+      const radius = unit.areaRadius();
+
+      const ux = this.x(unit.tile());
+      const uy = this.y(unit.tile());
+      const dx = tx - ux;
+      const dy = ty - uy;
+
+      // Faster angle approximation or just use atan2 (it's called once per nearby city)
+      const angle = Math.atan2(dy, dx);
+      const uid = unit.id();
+
+      // Simplified noise with fewer sin calls
+      const noise =
+        1 + 0.18 * Math.sin(angle * 3 + uid) + 0.12 * Math.sin(angle * 5 - uid);
+
+      const irregularRadius = radius * noise;
+
+      if (dist <= irregularRadius) {
+        // Linear fade is faster than Math.pow
+        const fade = 1 - dist / irregularRadius;
+        const d = unit.density() * fade;
+
+        if (d > maxDensity) {
+          maxDensity = d;
+          maxUnit = unit;
+        }
+      }
+    }
+    const result = { density: maxDensity, unit: maxUnit };
+    this.urbanizationCache.set(tile, result);
+    return result;
+  }
 }
diff --git a/src/core/game/PlayerImpl.ts b/src/core/game/PlayerImpl.ts
index e2005f248..225463c7a 100644
--- a/src/core/game/PlayerImpl.ts
+++ b/src/core/game/PlayerImpl.ts
@@ -948,6 +948,12 @@ export class PlayerImpl implements Player {
     ) {
       return false;
     }
+    if (
+      unit.type() === UnitType.City &&
+      this.troops() < 25_000 * unit.level()
+    ) {
+      return false;
+    }
     if (unit.owner() !== this) {
       return false;
     }
@@ -957,6 +963,9 @@ export class PlayerImpl implements Player {
   upgradeUnit(unit: Unit) {
     const cost = this.mg.unitInfo(unit.type()).cost(this.mg, this);
     this.removeGold(cost);
+    if (unit.type() === UnitType.City) {
+      this.removeTroops(25_000 * unit.level());
+    }
     unit.increaseLevel();
     this.recordUnitConstructed(unit.type());
   }
diff --git a/src/core/game/UnitImpl.ts b/src/core/game/UnitImpl.ts
index 67791b458..42ef9fa3e 100644
--- a/src/core/game/UnitImpl.ts
+++ b/src/core/game/UnitImpl.ts
@@ -42,6 +42,8 @@ export class UnitImpl implements Unit {
   private _trajectoryIndex: number = 0;
   private _trajectory: TrajectoryTile[];
   private _deletionAt: number | null = null;
+  private _areaRadius: number = 0;
+  private _age: number = 0;
 
   constructor(
     private _type: UnitType,
@@ -69,6 +71,10 @@ export class UnitImpl implements Unit {
       "loaded" in params ? (params.loaded ?? undefined) : undefined;
     this._trainType = "trainType" in params ? params.trainType : undefined;
 
+    if (this._type === UnitType.City) {
+      this._areaRadius = 7;
+    }
+
     switch (this._type) {
       case UnitType.Warship:
       case UnitType.Port:
@@ -142,6 +148,8 @@ export class UnitImpl implements Unit {
       hasTrainStation: this._hasTrainStation,
       trainType: this._trainType,
       loaded: this._loaded,
+      areaRadius: this.areaRadius() || undefined,
+      age: this.age() || undefined,
     };
   }
 
@@ -164,7 +172,7 @@ export class UnitImpl implements Unit {
   }
 
   setTroops(troops: number): void {
-    this._troops = troops;
+    this._troops = Math.max(0, troops);
   }
   troops(): number {
     return this._troops;
@@ -451,6 +459,9 @@ export class UnitImpl implements Unit {
 
   increaseLevel(): void {
     this._level++;
+    if (this.type() === UnitType.City) {
+      this._areaRadius += 2; // Expand city area with level
+    }
     if ([UnitType.MissileSilo, UnitType.SAMLauncher].includes(this.type())) {
       this._missileTimerQueue.push(this.mg.ticks());
     }
@@ -459,6 +470,9 @@ export class UnitImpl implements Unit {
 
   decreaseLevel(destroyer?: Player): void {
     this._level--;
+    if (this.type() === UnitType.City) {
+      this._areaRadius = Math.max(7, this._areaRadius - 2);
+    }
     if ([UnitType.MissileSilo, UnitType.SAMLauncher].includes(this.type())) {
       this._missileTimerQueue.pop();
     }
@@ -483,4 +497,32 @@ export class UnitImpl implements Unit {
       this.mg.addUpdate(this.toUpdate());
     }
   }
+
+  areaRadius(): number {
+    return Math.max(1, this._areaRadius);
+  }
+
+  age(): number {
+    return this._age;
+  }
+
+  incrementAge(): void {
+    this._age++;
+    // We update the unit every 10 ticks to avoid excessive network traffic
+    if (this._age % 10 === 0) {
+      this.mg.addUpdate(this.toUpdate());
+    }
+  }
+
+  density(): number {
+    if (this.type() !== UnitType.City) return 0;
+    // Growth rate: smaller cities fill up faster (lower time constant)
+    // One tick is 100ms, so 200 * Level is 20 * Level seconds to reach ~63% of potential
+    const timeConstant = 200 * this._level;
+    // Potential density: larger cities can become much denser (higher cap)
+    const maxDensity = 0.5 + (this._level - 1) * 0.5;
+
+    const density = (1 - Math.exp(-this._age / timeConstant)) * maxDensity;
+    return Math.max(0, density) || 0;
+  }
 }