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 to attack oncoming Vectoids. Success is not based solely on the types of tower you select, but where you place them and what upgrade and bonus options you take.
Select a map from the list, click on Load Map then 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 wave you can build some more towers and/or upgrade existing ones. Press Send the Vectoids again to release the next wave.