diff --git a/package-lock.json b/package-lock.json
index aef93b9..4609fa7 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10,6 +10,7 @@
       "devDependencies": {
         "@sveltejs/vite-plugin-svelte": "^3.1.2",
         "@tsconfig/svelte": "^5.0.4",
+        "immer": "^10.1.1",
         "svelte": "^4.2.19",
         "svelte-check": "^4.0.4",
         "tslib": "^2.7.0",
@@ -877,6 +878,16 @@
         "node": "^8.16.0 || ^10.6.0 || >=11.0.0"
       }
     },
+    "node_modules/immer": {
+      "version": "10.1.1",
+      "resolved": "https://registry.npmjs.org/immer/-/immer-10.1.1.tgz",
+      "integrity": "sha512-s2MPrmjovJcoMaHtx6K11Ra7oD05NT97w1IC5zpMkT6Atjr7H8LjaDd81iIxUYpMKSRRNMJE703M1Fhr/TctHw==",
+      "dev": true,
+      "funding": {
+        "type": "opencollective",
+        "url": "https://opencollective.com/immer"
+      }
+    },
     "node_modules/is-reference": {
       "version": "3.0.2",
       "resolved": "https://registry.npmjs.org/is-reference/-/is-reference-3.0.2.tgz",
diff --git a/package.json b/package.json
index 7fe91a0..67a12e3 100644
--- a/package.json
+++ b/package.json
@@ -12,6 +12,7 @@
   "devDependencies": {
     "@sveltejs/vite-plugin-svelte": "^3.1.2",
     "@tsconfig/svelte": "^5.0.4",
+    "immer": "^10.1.1",
     "svelte": "^4.2.19",
     "svelte-check": "^4.0.4",
     "tslib": "^2.7.0",
diff --git a/src/App.svelte b/src/App.svelte
index be2cd56..72933a1 100644
--- a/src/App.svelte
+++ b/src/App.svelte
@@ -1,8 +1,58 @@
 <script lang="ts">
+    import { onMount } from "svelte";
+
+    import moves from "./moves";
+    import update from "./update";
+    import village from "./village";
+
+
+    onMount(() => {
+        let frame: number;
+
+        function loop(timestamp: number) {
+            frame = requestAnimationFrame(loop);
+            update(timestamp);
+        }
+        frame = requestAnimationFrame(loop);
+
+        return () => {
+            cancelAnimationFrame(frame);
+        }
+    });
+
+
+    function upgradeBuilding(id: number) {
+        moves.upgradeBuilding(id);
+    }
 </script>
 
 <main>
+    <div>
+        <ul>
+            <li>Wood: { Math.floor($village.resources.wood) }</li>
+            <li>Stone: { Math.floor($village.resources.stone) }</li>
+            <li>Iron: { Math.floor($village.resources.iron) }</li>
+            <li>Food: { Math.floor($village.resources.food) }</li>
+        </ul>
+    </div>
+    <div class="buildings">
+        { #each $village.buildings as building }
+        <div>
+            <p>{ building.name }</p>
+            <p>Level: { building.level }</p>
+            <p>
+                <button on:click={ () => upgradeBuilding(building.id) }>
+                    Upgrade
+                </button>
+            </p>
+        </div>
+        { /each }
+    </div>
 </main>
 
 <style>
+    .buildings {
+        display: flex;
+        gap: 1em;
+    }
 </style>
diff --git a/src/app.css b/src/app.css
index 865860a..0baf90b 100644
--- a/src/app.css
+++ b/src/app.css
@@ -4,8 +4,8 @@
     font-weight: 400;
 
     color-scheme: light dark;
-    color: rgba(255, 255, 255, 0.87);
-    background-color: #242424;
+    color: hsla(0, 0%, 100%, 0.87);
+    background-color: hsl(0, 0%, 14%);
 
     font-synthesis: none;
     text-rendering: optimizeLegibility;
diff --git a/src/buildings.ts b/src/buildings.ts
new file mode 100644
index 0000000..07b77dc
--- /dev/null
+++ b/src/buildings.ts
@@ -0,0 +1,62 @@
+import type { VillageState } from "./village";
+
+
+export default {
+    'woodcutter': {
+        name: 'Woodcutter',
+        level: 1,
+        cost: (level: number) => {
+            return {
+                wood: level * 10,
+                stone: level * 10,
+                iron: level * 10,
+                food: 0,
+            };
+        },
+        behavior: {
+            production: (V: VillageState, self: any, delta: number) => {
+                const outputPerMinute = 5 * (self.level * self.level);
+                const outputPerMilisecond = outputPerMinute /  60.0 / 1000.0;
+                V.resources.wood += outputPerMilisecond * delta;
+            }
+        }
+    },
+    'mine': {
+        name: 'Mine',
+        level: 1,
+        cost: (level: number) => {
+            return {
+                wood: level * 10,
+                stone: level * 10,
+                iron: level * 10,
+                food: 0,
+            };
+        },
+        behavior: {
+            production: (V: VillageState, self: any, delta: number) => {
+                const outputPerMinute = 5 * (self.level * self.level);
+                const outputPerMilisecond = outputPerMinute /  60.0 / 1000.0;
+                V.resources.iron += outputPerMilisecond * delta;
+            }
+        }
+    },
+    'pit': {
+        name: 'Pit',
+        level: 1,
+        cost: (level: number) => {
+            return {
+                wood: level * 10,
+                stone: level * 10,
+                iron: level * 10,
+                food: 0,
+            };
+        },
+        behavior: {
+            production: (V: VillageState, self: any, delta: number) => {
+                const outputPerMinute = 5 * (self.level * self.level);
+                const outputPerMilisecond = outputPerMinute /  60.0 / 1000.0;
+                V.resources.stone += outputPerMilisecond * delta;
+            }
+        }
+    },
+};
diff --git a/src/main.ts b/src/main.ts
index 4d67e2a..bba7235 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,8 +1,10 @@
 import './app.css'
 import App from './App.svelte'
 
-const app = new App({
-  target: document.getElementById('app')!,
-})
 
-export default app
+const app = new App({
+    target: document.getElementById('app')!,
+});
+
+
+export default app;
diff --git a/src/moves/index.ts b/src/moves/index.ts
new file mode 100644
index 0000000..f5f3061
--- /dev/null
+++ b/src/moves/index.ts
@@ -0,0 +1,31 @@
+import { produce } from 'immer';
+
+import village, { type VillageState } from '../village';
+import upgradeBuilding from './upgradeBuilding';
+
+
+// Encapsulates a move function into a store update, where the data is made
+// immutable by immer. This allows moves to be much, much simpler to write,
+// while ensuring that states are always immutable. We'll benefit from this
+// when we want to implement an undo option.
+export function makeMove(move: (...args: any[]) => boolean) {
+    // Create a new function.
+    return (...args: any[]) => {
+        let result: boolean = false;
+
+        // That updates the game state.
+        village.update(state => {
+            // With an immutable state.
+            return produce(state, (G: VillageState) => {
+                result = move(G, ...args);
+            });
+        });
+
+        return result;
+    };
+}
+
+
+export default {
+    upgradeBuilding: makeMove(upgradeBuilding),
+};
diff --git a/src/moves/upgradeBuilding.ts b/src/moves/upgradeBuilding.ts
new file mode 100644
index 0000000..3843b66
--- /dev/null
+++ b/src/moves/upgradeBuilding.ts
@@ -0,0 +1,29 @@
+import type { VillageState } from "../village";
+
+
+export default function upgradeBuilding(V: VillageState, buildingId: number) {
+    const building = V.buildings.find(b => b.id === buildingId);
+    if (!building) {
+        return false;
+    }
+
+    const cost = building.cost(building.level);
+
+    if (
+        cost.wood > V.resources.wood
+        || cost.stone > V.resources.stone
+        || cost.iron > V.resources.iron
+        || cost.food > V.resources.food
+    ) {
+        return false;
+    }
+
+    V.resources.wood -= cost.wood;
+    V.resources.stone -= cost.stone;
+    V.resources.iron -= cost.iron;
+    V.resources.food -= cost.food;
+
+    building.level++;
+
+    return true;
+}
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000..1096336
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,21 @@
+export interface Cost {
+    wood: number;
+    stone: number;
+    iron: number;
+    food: number;
+}
+
+
+export interface BuildingSource {
+    name: string;
+    level: number;
+    cost: (level: number) => Cost;
+    behavior: {
+        production?: Function;
+    };
+}
+
+
+export interface Building extends BuildingSource {
+    id: number;
+}
diff --git a/src/update.ts b/src/update.ts
new file mode 100644
index 0000000..14e8f45
--- /dev/null
+++ b/src/update.ts
@@ -0,0 +1,29 @@
+import { produce } from 'immer';
+
+import village, { type VillageState } from "./village";
+
+
+let lastFrame: number;
+
+
+export default function update(timestamp: number) {
+    if (!lastFrame) {
+        lastFrame = timestamp;
+        return;
+    }
+
+    const delta = timestamp - lastFrame;
+
+    village.update(state => {
+        return produce(state, (V: VillageState) => {
+            V.buildings.forEach(b => {
+                if (b.behavior.production) {
+                    b.behavior.production(V, b, delta);
+                }
+            });
+            return V;
+        });
+    });
+
+    lastFrame = timestamp;
+}
diff --git a/src/village.ts b/src/village.ts
new file mode 100644
index 0000000..f8726b8
--- /dev/null
+++ b/src/village.ts
@@ -0,0 +1,44 @@
+import { writable } from "svelte/store";
+import buildings from "./buildings";
+import type { Building } from "./types";
+
+
+export interface VillageState {
+    buildings: Building[];
+    resources: {
+        wood: number;
+        stone: number;
+        iron: number;
+        food: number;
+        culture: number;
+    }
+}
+
+let uid = 0;
+
+const village = writable<VillageState>({
+    buildings: [
+        { ...buildings.woodcutter, id: uid++ },
+        { ...buildings.woodcutter, id: uid++ },
+        { ...buildings.woodcutter, id: uid++ },
+        { ...buildings.woodcutter, id: uid++ },
+        { ...buildings.mine, id: uid++ },
+        { ...buildings.mine, id: uid++ },
+        { ...buildings.mine, id: uid++ },
+        { ...buildings.mine, id: uid++ },
+        { ...buildings.pit, id: uid++ },
+        { ...buildings.pit, id: uid++ },
+        { ...buildings.pit, id: uid++ },
+        { ...buildings.pit, id: uid++ },
+    ],
+    resources: {
+        wood: 100,
+        stone: 100,
+        iron: 100,
+        food: 0,
+        culture: 0,
+    },
+});
+
+
+export default village;