Fun, but could be improved
I can't lie, this is fun as hell to me and helped pass alot of time. There are a few things that I would improve upon though: more towers and zombie types. I found that I could camp on a level with only one base and one fully powered up sniper. I also never had to use more than one sniper, I found it a waste of survivers. In other words, this game is too easy. I'd add faster zombies, zombies that can take a few hits and other difficult ones. Maybe throw in some flamethrower and machine gun towers and you've just turned a fun game into an awesome one, one more advanced than a traditional tower defence!