The Underworld Castle is a gigantic and twisted palace found in the Underworld ruled by Hades and it is the home of Medusa. It is the main location for the ground mission of Chapter 9: Medusa's Final Battle of Kid Icarus: Uprising. The castle is guarded by the Underworld Gatekeeper.
There are statues of Twinbellows, Hewdraw and Pandora and a Hot Spring located in the main entrance.
There are doors corresponding to the 3 statues that transport Pit to sections of the levels that the bosses were located at.
After the bosses have been defeated and the statues have crumbled, Pit advances to the next section of the level which features a cave complex complete with flamethrowers attached to the walls.
After Pit has moved past the caves he comes into a large corridor that seemingly has no way across until Pit steps on a switch which makes lights appear which Pit can walk in the middle to advance to the next section.
This looming castle appears at the time of Medusa's arrival, eclipsing all in its shadow. The goddess of the Underworld calls this foul fortress her home, lurking behind impregnable walls guarded by the Underworld Gatekeeper.