This was great, don't change anything.
Because you're missing a lot of things that're almost requisite for most flash defense games, real close to EVERYONE who plays this game on Newgrounds is going to try and give you advice, or spit out all their ideas like you were asking for it. They're wrong.
Playing this game was fantastic. Short enough to stay interesting, but long enough for fulfillment in completion, a good range of weapons, excellent graphics, different kinds of zombies without going too far and making, like, supermutants with wings or whatever, and realistic physics. Of course, it can be improved, but please don't warp the game if you sequel it. Just take what you've got and make it better, and more of what it is. It seriously does not need any more features.