I didn't figure out how to use the upgrades until after I beat the game... I never lost a level. So yeah, could have been a bit more difficult if I beat it w/o any upgrades. But it was a pretty good game, I must admit.
Twasn't bad, although the level where you're saving the hostage by shooting the rope off his hands made me laugh... That would so kill him, but this is a game so what evers.
I don't know what people are talking about when they say its hard, it was pretty easy.... Just make some of those steel boxes, pile up some cannons in the center, and use the Satan stomp spell thing.
 "Colony"){kind=icon}
 "Phage Wars 2"){kind=icon}
 "S.W.A.T. 2 - Tactical Sniper"){kind=icon}
 "The Tower"){kind=icon}