Very neat. It would be nice if the upgrade and turret building menus didn't persist, leaving them to hinder view. The development of the troops seems further than that of the zombies because there are multiple units as well as upgrades to them. Thus, the zombies feel unexplored. If there were some unique and interesting zombie types to throw into the mix, instead of just adjusting their stats, I think the game would definitely feel more like you are playing both sides. The early game is very hard to get zombies that are fast enough to make before the time runs out; this makes it feel as though there is really only one way to play the game, and a tower defense game should have multiple outlets to achieve success. Overall fun to play and a very unique concept to be explored.
P.S. A brief tutorial as a part of the game helps keep it all together as opposed to reading the description for how the game is played.