I stopped building anti air towers as they are just a complete waste if cash. The air units come from random places. normally out of the tower's range. Even if you managed to luckily guess where they will appear, the air units are so fast that the tower doesn't do anything. With the money I save from not buying anti air, I just repair and buy better units rather than wasting money on a useless tower.
 "Realm Grinder"){kind=icon}
 "Idle Raiders"){kind=icon}
 "Canyon Defense 2"){kind=icon}