Not bad. The later levels get pretty laggy, I assume because of the growing number of zombies and turret missles. The game concept has been done before, but I still think this was executed well and there was enough variation in the enemies and upgrades to play through the entire game without getting bored. The difficulty was pretty fair - if anything I might say a bit too easy.
Short, yet entertaining enough to beat in one sitting. Nicely done; keep up the good work.