wow are u serious its spreading???? strat. get chains, and upgrade them, for air 1 just get a lvl 2 AA tower and it hold off em all, a lvl 1 hold em all also, but upgrading is easy and cheap.
and about the difficulty, I kno that lvl 8 is way too hard, and lvl 9 is also way too hard (goes from 600hp - fort armor (lvl 8), to 1100hp normal armor(lvl 9))
im workin on it. balancing is always the biggest problem in TDs