Cool! A neat game to let run in the background (I'm on round 24 as I write this), and for this reason I think the balancing could be better. Maybe make it a little more difficult? Instead of adding more zombies you could also increase their health. I like the style of the towers and I think that maybe the rest of the game could've been done in the same style because the zombies and player feel really out of place in my opinion. Anyways, just made it to round 31. Good job!

Simple tower defense game, which is made harder/more interesting by limited space available for towers, and those not being destructible. No great adherence to the theme, though. I made it to wave 44.