Castle Hell is the royal castle in the Underworld, where Malladus Uno and the Demon Royal Family make their home. At the top of this castle is an entrance to the Spirit World. The main group had to fight the Demon Train above this ocean in Operation: ANCESTOR. Since Malladus was defeated and the Grim Reaper became Demon King, Grim now resides in the castle.
When Ganondorf rises to power in Legend of the Eight Firstborn, the castle rises above the Mortal World, in the place of Delightful Mansion, and falls under the will of Ganondorf. It is the 55th stage in Legend of the Eight Firstborn, and the final stage.