First off, the game looks nice! Although, it would've been nice if you used something else than the default font, because it looks unprofessional if you don't. Also, the lose screen is just a black screen.
The progression doesn't really work, since the enemies spawn all at once, & not in waves. Also, you had a cap on the amount of towers to give difficulty, but there's no real strategy to it. You just put down all your towers at the beginning, & wait until you lose. Maybe a money system in between waves instead could've solved it.
Sorry if I came across a bit harsh, but overall, I'd say your game was okay.