Very nice
Excellent atmosphere and a good solid game balance. I think the game is so well refined that it leaves the door open to add a bit more on; I would love to see a sequel. Perhaps add a bit of frills to the defenses, like giving different rescued people different weapons, or something. Also, could use another dimension in the actual defense other than "CLICK LIKE CRAZY ON THE ZOMBIES!!!"
One of the better defense games I've seen.