Not bad
I actually liked it. It may be because I have an afinity for all things zombie related, but this wasn't bad at all. I do have some suggestions tho:
First of all, its way too hard. With the automatic reloading and increase rush of zombies, it's impossible to last more than 30 seconds. I would give it more of a "Defend your Castle" feel (if you don't know what game that is, Google search it). Give it stages that you have to pass that gradually become harder, and put some upgrades in so that the player doesn't become bored. I'd suggest making new enemies with each round (bigger, tougher, faster zombies, maybe an occasional "boss" zombie). I would also make each zombie worth a certain amount of money that you can use between stages to upgrade your weapons and defenses. Maybe even find occasional "survivors" that can help you fend off the horde.
Second, just a few minor cosmetic details: I noticed that the zombies that are supposed to be further back, and thus show by being elevated over the zombies that are closer to the screen, actually walk on top of the other zombies. It makes it look like they're walking on each others shoulders. Just something small that I find a little annoying. I'd also add some music and some more sound effects, like zombies grunting, etc.
Last thing: during the time it took me to write this short novel (and I only spend the time doing it because I think you have the potential for a really fun game here. This is all constructive, not meant to offend), the zombie horde has continued to grow in mass, and is literally bogging down my computer. You might want to think about sending it back to the start screen when the player dies.
That's about it. Overall, you've got the potential for a really great game here. I hope you work on it a little more and heed my advice. Good luck!