You don't actually bring the flying fortresses down, just kill everyone there, and retrieve a couple items for quest solutions, for platform upgrades.
There is a ten piece equipment set in each of the fortresses for Stone (Archmage set), Raze (Hunter's set - ranger) and Kali (Scorpion set - warrior). If you mindread those three bosses, as well as Xanlosch (in Stone's fortress), you can collect the four piece Ulthring's set.
There is an achievement when you complete all of the fortresses, but nothing for each individual one.
In Stone's arena, the only other thing there to find is the
invoker jewel.
