This is gonna be one negative post, but here are some things I think could use some improvement.
I played through 100 waves, though around wave 30 I already felt like I had seen everything. The difficulty never really ramps up, the waves got longer but not any more difficult. I spent the last 40 waves walking around and deploying turrets without firing a single shot myself.
The upgrade system could use some balancing, right now it feels like maxing damage first is the clear best choice, while health is pretty useless.
Turrets should actively look for better targets instead of switching only when their last target has died. This is especially an issue at the beginning of a new wave, when all the turrets lock onto the enemy that was spawned first, and if that happens to be on the other side of the screen then they'll waste a lot of time doing nothing until that enemy is dead.
Some other things to note, the option to buy ammo should also get cheaper if you're only missing less than one clip instead of letting you waste money. Also, replenishing all your ammo when you're all out also reloads instantly. Not sure if this is intentional, but it lets you deploy turrets a lot faster than usual.
Finally, I started experiencing some framerate issues after wave 90 or so, with CPU use at a constant 100%. Might want to check if everything is cleaned up properly when destroying objects and such.