Once the Pure Crystal was cracked and became the Dark Crystal, the Mystics fled to their valley, and the Skeksis took rule of the Castle. Over time, the Castle became dark and foreboding, looking as evil as its rulers truly were.
It is within the Castle's Crystal Chamber that the Gelflings Jen and Kira healed the Dark Crystal and reunited the two species into one. When the Crystal was healed, the Castle shed its dark covering and revealed its true visage.
The castle's features include the Crystal Chamber above the Lake of Fire, the Chamber of Life laboratory, the Garthim Pit, the Throne Room, the Emperor's bedchambers, the Banquet Hall, the Teeth of Skreesh (a massive carving in the shape of a gargoyle's face near the bottom of the ravine that surrounds the Castle) and a maze of sewer tunnels.