diff --git a/eslint.config.js b/eslint.config.js
index 1d736e7..b64a17a 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -15,4 +15,18 @@ export default tsEslint.config(
       },
     },
   },
+  {
+    rules: {
+      "@typescript-eslint/restrict-template-expressions": [
+        "error",
+        { allowNumber: true, allowNever: true },
+      ],
+    },
+  },
+  {
+    files: ["**/*.spec.*"],
+    rules: {
+      "@typescript-eslint/require-await": "off",
+    },
+  },
 );
diff --git a/src/lib/board.spec.ts b/src/lib/board.spec.ts
new file mode 100644
index 0000000..7cac5c2
--- /dev/null
+++ b/src/lib/board.spec.ts
@@ -0,0 +1,63 @@
+import t, { type Test } from "tap";
+import { Board } from "./board.ts";
+import { SquareState } from "./types.js";
+
+const createAndCheckBoard = (t: Test, serialized: string) => {
+  const board = new Board(serialized);
+  t.equal(board.serialize(), serialized);
+  return board;
+};
+
+void t.test("0x0 board", async (t) => {
+  const board = createAndCheckBoard(t, "");
+  t.throws(() => board.get(0, 0), { message: "Out of bounds: 0:0" });
+  t.throws(() => board.get(0, 1), { message: "Out of bounds: 0:1" });
+  t.throws(() => board.get(1, 0), { message: "Out of bounds: 1:0" });
+  t.throws(() => board.get(1, 1), { message: "Out of bounds: 1:1" });
+});
+
+void t.test("Empty 1x1 board", async (t) => {
+  const board = createAndCheckBoard(t, "_");
+  t.equal(board.get(0, 0), SquareState.Unoccupied);
+  t.throws(() => board.get(0, 1), { message: "Out of bounds: 0:1" });
+  t.throws(() => board.get(1, 0), { message: "Out of bounds: 1:0" });
+  t.throws(() => board.get(1, 1), { message: "Out of bounds: 1:1" });
+});
+
+void t.test("1x1 board with X", async (t) => {
+  const board = createAndCheckBoard(t, "X");
+  t.equal(board.get(0, 0), SquareState.X);
+  t.throws(() => board.get(0, 1), { message: "Out of bounds: 0:1" });
+  t.throws(() => board.get(1, 0), { message: "Out of bounds: 1:0" });
+  t.throws(() => board.get(1, 1), { message: "Out of bounds: 1:1" });
+});
+
+void t.test("Half-full 3x4 board", async (t) => {
+  const board = createAndCheckBoard(t, "XO_|O_X|OX_|_XO");
+  t.equal(board.get(0, 0), SquareState.X);
+  t.equal(board.get(0, 1), SquareState.O);
+  t.equal(board.get(0, 2), SquareState.Unoccupied);
+  t.throws(() => board.get(0, 3), { message: "Out of bounds: 0:3" });
+  t.equal(board.get(1, 0), SquareState.O);
+  t.equal(board.get(1, 1), SquareState.Unoccupied);
+  t.equal(board.get(1, 2), SquareState.X);
+  t.throws(() => board.get(1, 3), { message: "Out of bounds: 1:3" });
+  t.equal(board.get(2, 0), SquareState.O);
+  t.equal(board.get(2, 1), SquareState.X);
+  t.equal(board.get(2, 2), SquareState.Unoccupied);
+  t.throws(() => board.get(2, 3), { message: "Out of bounds: 2:3" });
+  t.equal(board.get(3, 0), SquareState.Unoccupied);
+  t.equal(board.get(3, 1), SquareState.X);
+  t.equal(board.get(3, 2), SquareState.O);
+  t.throws(() => board.get(3, 3), { message: "Out of bounds: 3:3" });
+  t.throws(() => board.get(4, 0), { message: "Out of bounds: 4:0" });
+  t.throws(() => board.get(4, 1), { message: "Out of bounds: 4:1" });
+  t.throws(() => board.get(4, 2), { message: "Out of bounds: 4:2" });
+  t.throws(() => board.get(4, 3), { message: "Out of bounds: 4:3" });
+});
+
+void t.test("Throws error on incorrect serialized value", async (t) => {
+  t.throws(() => new Board("abc"), {
+    message: "Unsupported square character: a",
+  });
+});
diff --git a/src/lib/board.ts b/src/lib/board.ts
new file mode 100644
index 0000000..3b1dbe2
--- /dev/null
+++ b/src/lib/board.ts
@@ -0,0 +1,57 @@
+import { SquareState } from "./types.ts";
+import { unreachable } from "./utils.ts";
+
+export class Board {
+  private readonly state: SquareState[][];
+
+  constructor(serialized: string) {
+    this.state = serialized.split("|").map((line) =>
+      line.split("").map((char) => {
+        switch (char) {
+          case "_":
+            return SquareState.Unoccupied;
+          case "O":
+            return SquareState.O;
+          case "X":
+            return SquareState.X;
+          default:
+            throw new Error(`Unsupported square character: ${char}`);
+        }
+      }),
+    );
+  }
+
+  get(row: number, column: number) {
+    const result = this.state[row]?.[column];
+    if (!result) {
+      throw new Error(`Out of bounds: ${row}:${column}`);
+    }
+
+    return result;
+  }
+
+  serialize() {
+    return this.state
+      .map((row) =>
+        row
+          .map((squareState) => {
+            switch (squareState) {
+              case SquareState.Unoccupied:
+                return "_";
+              case SquareState.O:
+                return "O";
+              case SquareState.X:
+                return "X";
+              /* c8 ignore start */
+              default:
+                throw new Error(
+                  `Unsupported square state: ${unreachable(squareState)}`,
+                );
+              /* c8 ignore stop */
+            }
+          })
+          .join(""),
+      )
+      .join("|");
+  }
+}
diff --git a/src/lib/stub.spec.ts b/src/lib/stub.spec.ts
deleted file mode 100644
index e69de29..0000000
diff --git a/src/lib/stub.ts b/src/lib/stub.ts
deleted file mode 100644
index e69de29..0000000
diff --git a/src/lib/types.ts b/src/lib/types.ts
new file mode 100644
index 0000000..048c923
--- /dev/null
+++ b/src/lib/types.ts
@@ -0,0 +1,5 @@
+export enum SquareState {
+  Unoccupied = 1, // so that all SquareState values are truthy
+  X,
+  O,
+}
diff --git a/src/lib/utils.ts b/src/lib/utils.ts
new file mode 100644
index 0000000..52ea880
--- /dev/null
+++ b/src/lib/utils.ts
@@ -0,0 +1,4 @@
+// To simplify switches and provide some typecheck-time guarantees
+/* c8 ignore start */
+export const unreachable = (value: never) => value as unknown as string;
+/* c8 ignore stop */
diff --git a/tsconfig.build.json b/tsconfig.build.json
new file mode 100644
index 0000000..c8fea9c
--- /dev/null
+++ b/tsconfig.build.json
@@ -0,0 +1,8 @@
+{
+    "extends": "./tsconfig.json",
+    "exclude": ["src/**/*.spec.*"],
+    "compilerOptions": {
+      "module": "Preserve"
+    }
+  }
+  
\ No newline at end of file
diff --git a/tsconfig.json b/tsconfig.json
index 67d5a42..8dcaefa 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,10 +1,11 @@
 {
-    "extends": "@tsconfig/strictest",
-    "include": ["src/**/*"],
-    "compilerOptions": {
-      "target": "ES2020",
-      "module": "Preserve",
-      "forceConsistentCasingInFileNames": true
-    }
+  "extends": "@tsconfig/strictest",
+  "include": ["src/**/*"],
+  "compilerOptions": {
+    "allowImportingTsExtensions": true,
+    "noEmit": true,
+    "target": "ES2020",
+    "module": "NodeNext",
+    "forceConsistentCasingInFileNames": true
   }
-  
\ No newline at end of file
+}