Use the mouse to buy towers and place them around the path. If the enemies reach the cave, you will lose a life. Bombers can break rocks creating shortcuts for the enemies. If you need time to think, there is a pause button which will still allow you to place towers.
Tips: Every 10 levels the enemy will become stronger. Bombers move faster after each wave. Cannon towers deal extra damage against ogres, while shotguns do less damage against ogres.