diff --git a/src/shared/array-utils.spec.ts b/src/shared/array-utils.spec.ts
new file mode 100644
index 0000000..ffa37b4
--- /dev/null
+++ b/src/shared/array-utils.spec.ts
@@ -0,0 +1,14 @@
+import t from "tap";
+import { repeat, sequence } from "./array-utils.ts";
+
+void t.test("repeat", async (t) => {
+  t.matchOnlyStrict(repeat("x", 0), []);
+  t.matchOnlyStrict(repeat("x", 1), ["x"]);
+  t.matchOnlyStrict(repeat("x", 5), ["x", "x", "x", "x", "x"]);
+});
+
+void t.test("sequence", async (t) => {
+  t.matchOnlyStrict(sequence(0), []);
+  t.matchOnlyStrict(sequence(1), [0]);
+  t.matchOnlyStrict(sequence(5), [0, 1, 2, 3, 4]);
+});
diff --git a/src/shared/board.spec.ts b/src/shared/board.spec.ts
index 1afe1d7..2b027ef 100644
--- a/src/shared/board.spec.ts
+++ b/src/shared/board.spec.ts
@@ -89,20 +89,6 @@ void t.test("Serialize / deserialize", async (t) => {
     t.throws(() => Board.fromSerialized("abc"), {
       message: "Unsupported square character: a",
     });
-
-    {
-      const board = new Board([[undefined as any]]);
-      t.throws(() => board.serialize(), {
-        message: "Unsupported square state: undefined",
-      });
-    }
-
-    {
-      const board = new Board([["test" as any]]);
-      t.throws(() => board.serialize(), {
-        message: "Unsupported square state: test",
-      });
-    }
   });
 });
 
diff --git a/src/shared/board.ts b/src/shared/board.ts
index 1d5f6d7..e7e6327 100644
--- a/src/shared/board.ts
+++ b/src/shared/board.ts
@@ -3,7 +3,7 @@ import { BoardType, SquareState, formatSquareState, parseSquareState } from "./d
 
 export class Board implements BoardType {
   // State should be immutable
-  constructor(
+  private constructor(
     private readonly state: SquareState[][],
     private serialized?: string,
   ) {}
@@ -61,20 +61,7 @@ export class Board implements BoardType {
   }
 
   private forceSerialize() {
-    return this.state
-      .map((row) =>
-        row
-          .map((squareState) => {
-            const char = formatSquareState(squareState);
-            if (!char) {
-              throw new Error(`Unsupported square state: ${squareState}`);
-            }
-
-            return char;
-          })
-          .join(""),
-      )
-      .join("|");
+    return this.state.map((row) => row.map(formatSquareState).join("")).join("|");
   }
 
   serialize() {
diff --git a/src/shared/boardgame-state.spec.ts b/src/shared/boardgame-state.spec.ts
new file mode 100644
index 0000000..dc9acb5
--- /dev/null
+++ b/src/shared/boardgame-state.spec.ts
@@ -0,0 +1,238 @@
+import t, { type Test } from "tap";
+
+import { BoardgameState } from "./boardgame-state.ts";
+import { Player, SquareState } from "./datatypes.ts";
+
+const createAndCheckBoardgameState = (t: Test, serialized: string) => {
+  const boardgameState = BoardgameState.fromSerialized(serialized);
+  t.ok(boardgameState);
+  t.equal(boardgameState?.serialize(), serialized);
+  return boardgameState;
+};
+
+void t.test("Serialize / deserialize", async (t) => {
+  void t.test("Empty state", async (t) => {
+    t.equal(BoardgameState.fromSerialized(""), null);
+    t.equal(BoardgameState.fromSerialized(null), null);
+    t.equal(BoardgameState.fromSerialized(".."), null);
+  });
+
+  void t.test("1x1 state (incompletely serialized)", async (t) => {
+    const state = BoardgameState.fromSerialized("1x1");
+    t.ok(state);
+    t.equal(state?.serialize(), "1x1..");
+    t.equal(state?.rows, 1);
+    t.equal(state?.columns, 1);
+    t.equal(state?.currentPlayer, null);
+    t.equal(state?.currentPlayerName, "");
+    t.equal(state?.board, null);
+  });
+
+  void t.test("1x1 state with empty board", async (t) => {
+    const state = createAndCheckBoardgameState(t, "1x1..");
+    t.equal(state?.rows, 1);
+    t.equal(state?.columns, 1);
+    t.equal(state?.currentPlayer, null);
+    t.equal(state?.currentPlayerName, "");
+    t.equal(state?.board, null);
+  });
+
+  void t.test("1x1 board with started game", async (t) => {
+    const state = createAndCheckBoardgameState(t, "1x1.X._");
+    t.equal(state?.rows, 1);
+    t.equal(state?.columns, 1);
+    t.equal(state?.currentPlayer, Player.X);
+    t.equal(state?.currentPlayerName, "X");
+    t.equal(state?.board?.hasRow(0), true);
+    t.equal(state?.board?.hasSquare(0, 0), true);
+    t.equal(state?.board?.get(0, 0), SquareState.Unoccupied);
+    t.equal(state?.board?.hasSquare(0, 1), false);
+    t.equal(state?.board?.hasRow(1), false);
+  });
+
+  void t.test("1x2 board with first move", async (t) => {
+    const state = createAndCheckBoardgameState(t, "1x2.O._X");
+    t.equal(state?.rows, 1);
+    t.equal(state?.columns, 2);
+    t.equal(state?.currentPlayer, Player.O);
+    t.equal(state?.currentPlayerName, "O");
+    t.equal(state?.board?.get(0, 0), SquareState.Unoccupied);
+    t.equal(state?.board?.get(0, 1), SquareState.X);
+    t.equal(state?.board?.hasSquare(0, 2), false);
+    t.equal(state?.board?.hasRow(1), false);
+  });
+
+  void t.test("2x1 board with first move", async (t) => {
+    const state = createAndCheckBoardgameState(t, "2x1.O._|X");
+    t.equal(state?.rows, 2);
+    t.equal(state?.columns, 1);
+    t.equal(state?.currentPlayer, Player.O);
+    t.equal(state?.currentPlayerName, "O");
+    t.equal(state?.board?.get(0, 0), SquareState.Unoccupied);
+    t.equal(state?.board?.get(1, 0), SquareState.X);
+    t.equal(state?.board?.hasSquare(0, 1), false);
+    t.equal(state?.board?.hasRow(2), false);
+  });
+});
+
+void t.test("Empty state creation", async (t) => {
+  void t.test("0x0 board", async (t) => {
+    const state = BoardgameState.createWithoutBoard(0, 0);
+    t.equal(state.serialize(), "0x0..");
+    t.equal(state.rows, 0);
+    t.equal(state.columns, 0);
+    t.equal(state.currentPlayer, null);
+    t.equal(state.currentPlayerName, "");
+    t.equal(state.board, null);
+  });
+
+  void t.test("1x0 board", async (t) => {
+    const state = BoardgameState.createWithoutBoard(1, 0);
+    t.equal(state.serialize(), "1x0..");
+    t.equal(state.rows, 1);
+    t.equal(state.columns, 0);
+    t.equal(state.currentPlayer, null);
+    t.equal(state.currentPlayerName, "");
+    t.equal(state.board, null);
+  });
+
+  void t.test("1x1 board", async (t) => {
+    const state = BoardgameState.createWithoutBoard(1, 1);
+    t.equal(state.serialize(), "1x1..");
+    t.equal(state.rows, 1);
+    t.equal(state.columns, 1);
+    t.equal(state.currentPlayer, null);
+    t.equal(state.currentPlayerName, "");
+    t.equal(state.board, null);
+  });
+
+  void t.test("1x2 board", async (t) => {
+    const state = BoardgameState.createWithoutBoard(1, 2);
+    t.equal(state.serialize(), "1x2..");
+    t.equal(state.rows, 1);
+    t.equal(state.columns, 2);
+    t.equal(state.currentPlayer, null);
+    t.equal(state.currentPlayerName, "");
+    t.equal(state.board, null);
+  });
+
+  void t.test("Error handling", async (t) => {
+    t.throws(() => BoardgameState.fromSerialized("0.."), {
+      message: "Incorrect dimensions",
+    });
+  });
+});
+
+void t.test("withEmptyBoard", async (t) => {
+  void t.test("On 0x0 board", async (t) => {
+    const oldState = createAndCheckBoardgameState(t, "0x0..");
+    const newState = oldState?.withEmptyBoard();
+    t.equal(newState?.serialize(), "0x0.X.");
+    t.equal(newState?.rows, 0);
+    t.equal(newState?.columns, 0);
+    t.equal(newState?.currentPlayer, Player.X);
+    t.equal(newState?.currentPlayerName, "X");
+    t.equal(newState?.board?.serialize(), "");
+  });
+
+  void t.test("On 0x1 board", async (t) => {
+    const oldState = createAndCheckBoardgameState(t, "0x1..");
+    const newState = oldState?.withEmptyBoard();
+    t.equal(newState?.serialize(), "0x1.X.");
+    t.equal(newState?.rows, 0);
+    t.equal(newState?.columns, 1);
+    t.equal(newState?.currentPlayer, Player.X);
+    t.equal(newState?.currentPlayerName, "X");
+    t.equal(newState?.board?.serialize(), "");
+  });
+
+  void t.test("On 1x0 board", async (t) => {
+    const oldState = createAndCheckBoardgameState(t, "1x0..");
+    const newState = oldState?.withEmptyBoard();
+    t.equal(newState?.serialize(), "1x0.X.");
+    t.equal(newState?.rows, 1);
+    t.equal(newState?.columns, 0);
+    t.equal(newState?.currentPlayer, Player.X);
+    t.equal(newState?.currentPlayerName, "X");
+    t.equal(newState?.board?.serialize(), "");
+  });
+
+  void t.test("On 1x1 board", async (t) => {
+    const oldState = createAndCheckBoardgameState(t, "1x1..");
+    const newState = oldState?.withEmptyBoard();
+    t.equal(newState?.serialize(), "1x1.X._");
+    t.equal(newState?.rows, 1);
+    t.equal(newState?.columns, 1);
+    t.equal(newState?.currentPlayer, Player.X);
+    t.equal(newState?.currentPlayerName, "X");
+    t.equal(newState?.board?.serialize(), "_");
+  });
+
+  void t.test("On 1x2 board", async (t) => {
+    const oldState = createAndCheckBoardgameState(t, "1x2..");
+    const newState = oldState?.withEmptyBoard();
+    t.equal(newState?.serialize(), "1x2.X.__");
+    t.equal(newState?.rows, 1);
+    t.equal(newState?.columns, 2);
+    t.equal(newState?.currentPlayer, Player.X);
+    t.equal(newState?.currentPlayerName, "X");
+    t.equal(newState?.board?.serialize(), "__");
+  });
+
+  void t.test("On 2x1 board", async (t) => {
+    const oldState = createAndCheckBoardgameState(t, "2x1..");
+    const newState = oldState?.withEmptyBoard();
+    t.equal(newState?.serialize(), "2x1.X._|_");
+    t.equal(newState?.rows, 2);
+    t.equal(newState?.columns, 1);
+    t.equal(newState?.currentPlayer, Player.X);
+    t.equal(newState?.currentPlayerName, "X");
+    t.equal(newState?.board?.serialize(), "_|_");
+  });
+});
+
+void t.test("withMove", async (t) => {
+  void t.test("On 2x3 board", async (t) => {
+    const oldState = createAndCheckBoardgameState(t, "2x3.X.___|___");
+
+    const firstMoveState = oldState?.withMove(0, 1);
+    t.equal(firstMoveState?.serialize(), "2x3.O._X_|___");
+    t.equal(firstMoveState?.rows, 2);
+    t.equal(firstMoveState?.columns, 3);
+    t.equal(firstMoveState?.currentPlayer, Player.O);
+    t.equal(firstMoveState?.currentPlayerName, "O");
+    t.equal(firstMoveState?.board?.serialize(), "_X_|___");
+
+    const secondMoveState = firstMoveState?.withMove(1, 2);
+    t.equal(secondMoveState?.serialize(), "2x3.X._X_|__O");
+    t.equal(secondMoveState?.rows, 2);
+    t.equal(secondMoveState?.columns, 3);
+    t.equal(secondMoveState?.currentPlayer, Player.X);
+    t.equal(secondMoveState?.currentPlayerName, "X");
+    t.equal(secondMoveState?.board?.serialize(), "_X_|__O");
+  });
+
+  void t.test("Error handling", async () => {
+    const unstartedBoard = createAndCheckBoardgameState(t, "1x1..");
+    t.throws(() => unstartedBoard?.withMove(0, 0), {
+      message: "Game is not started",
+    });
+  });
+});
+
+void t.test("Sample usage scenario", async (t) => {
+  const noBoardState = BoardgameState.createWithoutBoard(2, 3);
+  t.equal(noBoardState.serialize(), "2x3..");
+
+  const gameStartedState = noBoardState.withEmptyBoard();
+  t.equal(gameStartedState.serialize(), "2x3.X.___|___");
+
+  const firstMoveState = gameStartedState.withMove(0, 1);
+  t.equal(firstMoveState.serialize(), "2x3.O._X_|___");
+
+  const secondMoveState = firstMoveState.withMove(1, 2);
+  t.equal(secondMoveState.serialize(), "2x3.X._X_|__O");
+
+  const thirdMoveState = secondMoveState.withMove(1, 0);
+  t.equal(thirdMoveState.serialize(), "2x3.O._X_|X_O");
+});
diff --git a/src/shared/datatypes.spec.ts b/src/shared/datatypes.spec.ts
index f2c70c5..a506cad 100644
--- a/src/shared/datatypes.spec.ts
+++ b/src/shared/datatypes.spec.ts
@@ -4,11 +4,15 @@ import {
   FinalOutcome,
   Player,
   SquareState,
+  formatPlayer,
+  formatSquareState,
   getDesiredFinalOutcomeByPlayer,
   getExpectedOutcomeByCurrentOutcome,
   getNextPlayer,
   getOccupiedStateByPlayer,
   getUndesiredFinalOutcomeByPlayer,
+  parsePlayer,
+  parseSquareState,
 } from "./datatypes.ts";
 
 // These unit tests are mostly useless because they're basically a tautology,
@@ -71,3 +75,35 @@ void t.test("getUndesiredFinalOutcomeByPlayer", async (t) => {
     message: "Unsupported player: test",
   });
 });
+
+void t.test("formatSquareState", async (t) => {
+  t.equal(formatSquareState(SquareState.Unoccupied), "_");
+  t.equal(formatSquareState(SquareState.X), "X");
+  t.equal(formatSquareState(SquareState.O), "O");
+  t.throws(() => formatSquareState("test" as unknown as SquareState), {
+    message: "Unsupported square state: test",
+  });
+});
+
+void t.test("parseSquareState", async (t) => {
+  t.equal(parseSquareState("_"), SquareState.Unoccupied);
+  t.equal(parseSquareState("X"), SquareState.X);
+  t.equal(parseSquareState("O"), SquareState.O);
+  t.equal(parseSquareState("test"), null);
+  t.equal(parseSquareState(undefined as unknown as string), null);
+});
+
+void t.test("formatPlayer", async (t) => {
+  t.equal(formatPlayer(Player.X), "X");
+  t.equal(formatPlayer(Player.O), "O");
+  t.throws(() => formatPlayer("test" as unknown as Player), {
+    message: "Unsupported player: test",
+  });
+});
+
+void t.test("parsePlayer", async (t) => {
+  t.equal(parsePlayer("X"), Player.X);
+  t.equal(parsePlayer("O"), Player.O);
+  t.equal(parsePlayer("test"), null);
+  t.equal(parsePlayer(undefined as unknown as string), null);
+});
diff --git a/src/shared/datatypes.ts b/src/shared/datatypes.ts
index 2f9e4ca..74ffa0d 100644
--- a/src/shared/datatypes.ts
+++ b/src/shared/datatypes.ts
@@ -1,4 +1,4 @@
-import { unreachableNull, unreachableString } from "./utils.ts";
+import { unreachableString } from "./utils.ts";
 
 export enum SquareState {
   Unoccupied = 1, // so that all SquareState values are truthy
@@ -128,7 +128,7 @@ export const formatSquareState = (squareState: SquareState) => {
     case SquareState.X:
       return "X";
     default:
-      return unreachableNull(squareState);
+      throw new Error(`Unsupported square state: ${unreachableString(squareState)}`);
   }
 };
 
diff --git a/src/shared/utils.ts b/src/shared/utils.ts
index 1bfb952..f8aac13 100644
--- a/src/shared/utils.ts
+++ b/src/shared/utils.ts
@@ -1,5 +1,2 @@
 // To simplify switches and provide some typecheck-time guarantees
 export const unreachableString = (value: never) => value as unknown as string;
-
-// eslint-disable-next-line @typescript-eslint/no-unused-vars -- We don't care about the value here, but we do care that from TS point of view it has type `never`
-export const unreachableNull = (_value: never) => null;