i prefer this over the 3D professional indie games
i love this sort of game where you build up a tower defense against waves of armies, the game is very solid and looks professional and the simple graphics add to the style, i've been searching around for professional versions of this game genre, but although they had pretty nice 3D graphics their basically the same as this. You should make many different skins for this game, such as spaceship, zombies, army men (WW2, future), dinosaurs, bugs, fish... cos all of them will kick ass... when you make something as awesome as this, that works superbly the only thing to do is make as many versions of it as possible. I really cant think of anything you can do to improve this current version