The aim of the game is to eliminate the Vectoid threat before they reach the end of the path. You do this by constructing towers along the path. Success is not based solely on the types of towers you select, but where you place them and what upgrade and bonus options you take. Different towers inflict different damage to different Vectoids. For example the red towers will do more damage to red Vectoids but reduced damage to green Vectoids. For more information move your mouse over the towers.