A really nice idea! I can see this expanded by giving you more abilities to unlock, which would also mean more towers to defend. If there is one thing I’d change, it’s the game area size / camera zoom. Right now the camera is a bit too close to the character, not giving you a lot of time between enemies entering your view and attacking your towers. Also, when the entire game is without a single sound or music and it suddenly screams out of your speakers, the player is up for a surprise. :P
Other than those, I really enjoyed it. I especially like the enemy design. Overall, nice work!