The invasion is on and you have limited resources to buy and build yourself a series of small towers in order to defend the larger tower. You can choose what kind of tower and whether or not you want to upgrade it, sell it for pennies on the dollar, or replace it with a tower that does something completely different. All of the different towers have different strengths and weaknesses.
Use your mouse and/or finger in order to interact with the game and the in-game menus.