Ahh, so polished!
Everything in this game is just right.
The music is a simple loop but after half an hour of listening it does not get annoying. Same thing with sounds. I am not sure how you manage to make that 'pop' sound so pleasant... or at least not irritable.
Graphics are just fine, perfect for such game. No need for anything more complex. Physics of the 'bullets' are ok. It's funny how they bounce.
Still, the best part is AI. You made a great piece of code here. I love how a turret remembers where it should aim, it's really small feature but imagine playing without it. Opponent's strategy is good also, quite challenging - it starts after 10th level but nothing wrong with few easy levels.
I loved gameplay also. The pace is perfect but speeding option is highly appreciated. It reminds me of Sims :)
As said, the game is perfect. It meets all game play expectations and makes it pleasurable to play. And re-play too!