The Castle of the Crystal is the imperial capital in the Alien world of Thra. It was a beautiful place, made entirely of all crystal and once the home of the ancient urSkeks, until the crystal cracked and made a division into Skeksis and Mystics; then the evil Skeksis took control of the castle and made it their home, banishing any other living being who wasn't a Skeksis away from the castle turf.
The castle is home of the Crystal. Originally the crystal laid hidden in a enclave surrounded by a rocky mountain, but all the beings and creatures of Thra knew it was there. When a Sun shone down the shaft of the Crystal, all would touch rock and feel the trembling from the Crystal. After the arrival of the urSkeks, they hollowed out the mountain around the Crystal and built a castle of lesser crystals around the great one.
After the world fell into darkness, the Castle became dark and foreboding. After the Crystal was healed, the Castle shed its darkened covering and revealed its true glorious visage.