[Micah]

Seconded on touch spells, although fists of iron isn't too horrible.

I'm gonna throw up Crumble for nomination though, 20 earth gems, it doesn't do much of anything to the units inside, it requires a powerful mage at earth 5, and probably worst of all if your siege force isn't hurting the walls to begin with crumble won't let you storm the castle, since it happens before the defenders repair the walls, so they get the gate closed before you can issue orders. I think you'd be better off alching those earth gems and buying 200 gold worth of troops, at least those are reusable for the next fort.