It's a fun game, thanks for the freebie. Though I seem to have found a glitch on level 3 that caused enemies to stop spawning after hitting 1427.
Other then that issue there are a couple of quality of life changes you might consider adding. It would be nice if the game returned to the same zoom level after picking an upgrade. Some volume controls would be nice. And finally labeling the attachment positions during upgrade choosing so you don't have to figure out the positions based on the orientation of the turret would be a bit helpful. But all in all those are minor additions that don't take away from what has so far been pretty neat.
Thanks again for giving it out for free.