Spent WAY too long playing this :D very good! one thing I noticed is that ground towers can still HIT air troops if they're targeting a ground troop that's behind the bird, so maybe you could add in collision layers so the air defense only hits air.
I had to stop myself at wave 200 in the end, and it was very fun to push the limits of the game :D
Also: Every building in this image is max level, and for some reason I'm quite proud of that