I think there might have been a bug of sorts as I was able to buy all the colors for my character after completing the first room? I'm not complaining, that yellow looks great :D
I would've liked more challenge, as I've managed to complete all the towers I've tried, before I figured out that powerups are bound to "E", not "B". Maybe some enemies with projectiles, falling traps, etc? :)
All in all, fun, hope you expand it.
BTW, we tried multiplayer, didn't manage to get in the same room, even though we typed in the same number, tried while logged in, while logged out... Oh well.