Cool. My initial strategy is to build a maze so that all except the flying units have to spend a lot of time going up and down instead of straight to the right. And a few swarm towers seem to keep the flying units at bay. So far I'm on level 33 and only one baddie has gotten through, a boss, and, even then, it was because I accidentally clicked "send next level" before the previous level was done.

Neat game, though.