The Wardens set a trap for Corypheus and bound him in a prison of their own making. Beneath the Free Marches, they carved out a series of caverns—a veritable maze—and enchanted them. Wards within wards, locks within locks, spells woven with the help of a powerful artifact they called "the key." All this to hold Corypheus.
It seems that even that wasn't enough. Warden-Commander Riannon writes in her private journals:
"Corypheus is too powerful. Nothing will hold him forever. The seals are already weakening. We must find a way to fortify them, and soon."
—From Janeka's research notes