Your mission is the same as in any other tower defense game: you must build and upgrade your towers to prevent the creeps getting past. You are limited by how much cash you can spend. Each creep you kill will give you more money. Saving up for bigger towers and upgrades will allow you to pack a heavier punch, but watch out - save for too long without getting new weaponry and you'll end up letting the creeps through. At the end of each level, interest is added to your cash reserves, so timing when to put a new tower down can make a lot of difference.