In the days of their first coming there were eighteen urSkeks, and they were full of vigor and the will to change and build. They hollowed out the mountain around the Crystal and built a castle of lesser crystals around the great Crystal. They made three walls composed of blocks of crystal, and above the Crystal they created a great portal with three sides. When the Suns moved over the Crystal, they stood framed in the portal, a triangle surrounding a circle.
The thought of the urSkeks was formed in triangles and pyramids, with circles and spheres placed within them; the castle was built as a poem to these shapes, and the movements of the Suns that shone through the portal completed the harmony.
But in the hearts of the shining urSkeks there struggled two beings living within one body – and for them, all things were divided so. Light and dark were the opposing spirits of the Universe to them; a mirror held to light should reflect dark, a mirror in the darkness should give light. And yet they knew so much of triangles and numbers, which combine and do not oppose.
The Areas Within the Castle of the Crystal:
This large chamber is used by the Skeksis for their grand and disgusting feasts. SkekAyuk the Gourmand doesn't cook anything himself. Instead, he commands a brigade of Podling slaves, who prepare and serve the Skeksis their meals. Common delicacies include roast nebrie, crawlies (served raw and very much alive), and boiled swamp weeds.
Chamber of Life
Once the laboratory of TekTih, the Chamber of Life is where skekTek the Scientist drains essence from captives and turns them into crystal slaves. The Chamber is located beneath the Crystal Chamber and it's filled with scientific apparatuses, alchemical equipment and animal cages. A system of levers, chains and gears enable the light of the Crystal to be refracted and reflected down the shaft and into the Chamber of Life through a viewing port that may be opened and closed via the same clockwork mechanisms.
The Crystal Chamber, also called the Ceremonial Chamber, is where the Dark Crystal is located, suspended above a shaft above the Lake of Fire. Surrounding the Crystal is a circle of 18 arcane symbols, one for each of the urSkeks that came to Thra during the first Great Conjunction. Above the Crystal is a triangular hatch in the roof of the Castle that can be opened to observe the movements of the triple suns.
The Emperor's Bedchambers are the third point in the triangle composed of this room, the Throne Room and the Banquet Hall. It is a regal room dominated by a massive bed. The other Skeksis have their own sleeping quarters, but these are much more sparse, resembling monks' cells rather than royal bedrooms.
A deep, dark pit where inactive Garthim are stored in-between their raids. The Garthim pit lies somewhere between the upper levels of the Castle and the lower tunnels, and is situated directly next to the Castle's central shaft.
Lake of Fire
The sewer tunnels are a maze of stalactites, stalagmites and dark tunnels. Scavengers and sightless creatures lurk, crawl and slither through the caves.
Teeth of Skreesh
The Teeth of Skreesh is a massive carving in the shape of a gargoyle's face. It's near the bottom of the ravine that surrounds the Castle. Effluent from the Castle's sewer runs out of the mouth of the carving. Apart from its inaccessibility, this opening is otherwise protected.
The Throne Room is the seat of the Skeksis Emperor's power. An opulent throne sits at one end of the room, where an ornate pattern of symbols (spirals, triangles and spheres) winds from the center of the chamber to the throne itself. A trapdoor in the floor is accessible via a system of chains, gears and pulleys. The black stone used during the ritual of Haakskeekah can be raised up from the floor. A crossed pair of basket-handled broadswords rests on the black stone. The stone itself was created by the Skeksis in a failed attempt to create a second Crystal of Truth.
After the Skeksis realized they could drain Gelfling life essence, skekTek the Scientest began experimentng with Gelfling from different clans and different draining techniques. Still trying to keep these horrible experiments a secret, he kept the victims of his failed and in-progress experiments out of the main Castle dungeons where the others might find them. Instead, he locked many of his victims in converted watch towers.