I love Tower Defenses, and this one is no different!
I do have a couple of minor points. It'd be nice to see how far along the wave is. I waited ~5 seconds until I discovered that the wave was already done :p.
And, since the gimmick of the game is turning on/off the towers, it might be nice to be able to do that on the map, instead of having to open the UI. It'd make the gameplay more fluid.