Which school of magic do you prefer and why?

Instead of the aforementioned wall of fire, it's better to use a force field – no one will get through it, while many creatures simply ignore the wall.

:D And how is that better? The purpose of the wall of fire is for creatures to pass through it. No one will attack a force field defended by just one fairy or angel, but hordes of Gorgons, Pit Lords, and 5 packs of 40 Griffons each will gladly charge, just to eat those fairies, and they're getting too cocky – they start showing up in the first week where they shouldn't. :D And who's ignoring it? Just a couple of monsters and archers. I don't attack them. :D