great engine
this engine works great, everything is nice about it...
here is what i think it needs,
speed control; controls the speed of which the wave flows
different enemies; for each level, different speeds of enemies, bosses
new graphics; the simplicity of the graphics are nice but you can easily make a sequel with this engine
backgrounds sound; some playing music with some controls of volume or different tracks for each level.
diverse sound fx; more sound fx for different towers, build sound fx, upgrade sound fx and enemy death...
so the game is nice, and you have a good engine to put towards making a sequel..
alright..