Little suggestions
1st off, could 'Send Next Wave' be mapped to the space bar? That would make it easier to play with one hand resting on the keyboard (and in one spot) and the other on the mouse; the "N" key is relatively far from the left hand side of the keyboard.
2nd off, how about shortcut keys for upgrade or sell tower?
All in all, a good example of a basic tower defense game, and a fun play with solid coding/interface.