I think you shoud fix tha balance. Once I build the 4 turrets I didn't have to click until the kities arrived. Perhaps the cost of the upgrades instead of 10, 20, 30, 40 should be 10, 40, 160, 640.

Kities arrive arround 1500 so I'd have two fully upgraded turrets and upgade other soon and hope the best.

Don't take my mumbers too serously, you should fine tune them.

you can resize the window to give your self breathing room, or if your towers are built more spawns closer to the bullets :D

great game design