Very interesting way to use 2 buttons! Great job!
It's a bit frustrating though when the tower misses the enemy by very small distance, so timing would be something that needs a bit of improvement.
Also, I'd suggest that you make initial enemies really easy, like 1 shot = kill just to hook us in basically :D it would be way more satisfying, give you nice and easy intro to how towers shoot and would give you motivation for the next round when harder enemies come at you.
But all in all, great job as I said! :)