Awesome game
Very good game, very fun to play. My only qualm was that I finished it wanting more. Adding a bit more complexity would make this a lot more enjoyable, or even just a storyline / bit more variety to the levels.
Not sure what I can suggest on that front, maybe creating some more troop diversity, or even better making the troops weaker and a lot cheaper so they are more dispensable. A sandbox mode where you can choose zombie ammounts / money / troop number / etc would also add a lot of replayability.
All in all this was very well conceived and I look forward to a sequel