Infinity cash, lives, and bonuses.
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.
Select and construct your towers along the path. When you are ready press “Send the Vectoids” at the top of the screen. When you have eliminated the first group you can build some more towers and/or upgrade existing ones then send the next group. Any Vectoids that reach the end of the path will cost you 1 life and will be teleported back to the start of the path. The game is over when you have completed 50 levels or you run out of lives.