The Citadel of Bone is the Skeleton King's grotesque aerial fortress that is both his main residence and his only base of operations. His pit of black ooze is in the ship and was used to create his Formless army to attack whatever planet he attacks in his quest to dominate the universe.
The Citadel of Bone is a giant spaceship made entirely out of bone, and the Skeleton King seems to be linked to the ship, because when the ship is attacked, the Skeleton King would feel incredible pain, which Chiro and the Super Robot Monkey Team along with their friend Jinmay used to their advantage when attacking the ship. The ship also has the power to regenerate when it is destroyed.
When it was destroyed in an asteroid belt, it slowly started piecing itself back together. The Citadel is also used to pierce the Skeleton King's Pit of Doom at the heart of planet Shuggazoom with one intention: to free the Dark One Worm, one of the legendary extradimensional fiends of great power known only as the Dark Ones.
Upon his resurrection, and after later the deathly "rewarding" demise of Valeena the Skull Sorceress, her master, the newly-reconstructed Skeleton King, summons forth from the grounds beneath the Pit of Doom a sinister-looking palace as his new Citadel of Bone as he soon leads his new Formless army in his war on planet Shuggazoom against Chiro and the Super Robot Monkey Team.
- As a nod to the Star Wars franchise, the ship looks like a Imperial Star Destroyer.