Really Great!
Good concept and good upgrades.
For those having trouble, keep you first rifle cause it's the fastest shooter. upgrade the bullet damage and you're good to go. Your base will get hit a few times but really try to save for the big turret. Get one, upgrade power, get second upgrade cooling and active time, then 3rd one, then fortification. Use the big bomb on 2, 3 or more gold wheel tanks, that'll hurt them the most.
There's only 20 levels! So it's not that long!