I mean think about it. The power of the Sim-Seru lets the room of mirrors transcend the power of time, so why not enable it to fly also? You have a gigantic clam floating around the place opening and closing its mouth like a Pac-Man and after you destroy the Absolute Fortress it causes the clam to fall to the earth and crush everyone, including Noa's parents.
The Floating Castle could just be another stationary Mist Generator.
Flying clam ftw?
