Pretty fun, here are a couple of notes:
-A few bugs with pathfinding, because people and zombies alike keep getting stuck
-Replay value to keep people playing, like new maps unlock after victory, or multiplayer, or rewards and increase difficulty on victory, or something as simple as a leaderboard
This game seems to have so much potential, with a bit more content this can turn into that addicting Indie game that you will play nonstop for a day or two...