During the game you will hold your defence with your turrets against the massive hordes of enemies. With every level you will have the use of new weapons at your command. It is up to you, commander, how to spend the resources: restore and upgrade your veteran units or to purchase some new ones.
Stay up-to-date on all the cool apps the kids are fiddling with these days. Or if you're a kid, help your parents log-in to their iTunes account. They would like apps, too.