Defend your castle from being destroyed by the invading waves of enemy forces. Buy troops and towers to fight the enemy.

Buy troops and towers, and upgrade them, to defend your castle.

Clicking each of your three buildings will bring up one of the three menus that allow you to buy Troops, Towers or Upgrades.
When you have purchased your initial defenses click on the arrow button in the lower right corner to start the waves of enemies. You can pause the game at any time to purchase more defenses by clicking the pause button in the lower right corner. When the Upgrade menu is open the game is also paused as the menu blocks the screen.
You can sell a tower and recover some of its cost by holding the S key while you click on the tower.