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Spiral Castle was at one time the seat of power for the fictional land of Prydain. During the period of time covered by Lloyd Alexander's Chronicles of Prydain, it had become one of many strongholds under the influence of Arawn Death-Lord, personally administered by Queen Achren.
The earliest story featuring Spiral Castle is "The Sword" from The Foundling and Other Tales from Prydain. It was the home of Rhitta, High King of Prydain and possessor of the sword Dyrnwyn, the land's most powerful weapon. The beginning of his reign saw much prosperity in the land, but through a few careless actions the king's guilty conscience quickly led him to personal tragedy. He became haunted by his crimes and paranoid of any threat to his rule.
To heighten security during the civil war that quickly engulfed the land, he had a vast labyrinth of tunnels dug beneath the castle as a final hiding place should the castle's defenses be overrun. It was because of these tunnels that the castle became called Spiral Castle. A few chambers were placed in the lower tunnels, and Rhitta would spend each night in a different one. His behavior became so unpredictable that no one besides the guards who watched over him knew where he was from one night to the next.
It was in one of these chambers that Rhitta met his death. Visited one night by the ghost of a shepherd he had killed unjustly, he tried to protect himself with the sword Dyrnwyn. However, Dyrnwyn could only be drawn by those "of noble worth" for purposes of good; as he was no longer worthy to draw the sword, when he tried doing so lightning from the sword killed him and all his guards instantly. A minor cave-in prevented others from discovering their bodies, and after Rhitta's disappearance Spiral Castle's importance to Prydain diminished.
Many years later, as told in The Book of Three, control of Spiral Castle had passed to Achren. Prince Gwydion and his companion Taran were captured by Arawn's forces and on Achren's orders they were taken to see her in her stronghold. Taran was imprisoned in the castle's dungeon, and there he met the Princess Eilonwy, Achren's ward, who was familiar with most of the castle's tunnels. She agreed to help Taran escape.
While Taran followed Eilonwy through the castle's tunnels, a section of floor collapsed and dropped Taran into a lower, buried, tunnel. Eilonwy followed him down, and eventually they discovered the chamber where King Rhitta met his end. In this chamber Taran discovered a small tunnel leading directly to the outside, in the face of a hill below the castle walls. He took a sword from the room and made his way out, followed by Eilonwy carrying the sword Dyrnwyn.
Removing Dyrnwyn proved to be Spiral Castle's undoing, for the sword's powers had affected the castle in such a way that it could no longer stand in the sword's absence. As Eilonwy left the tunnel with the sword, the castle collapsed, killing all its occupants in the process. Only the arch of the gate remained standing, and in only a matter of years the castle's ruins were overgrown with grass.