Spiral Castle sat atop a hill in the Ystrad Valley, west of the River Ystrad. The Castle lay close to Annuvin, for as Fflewddur Fflam told Taran, "Arawn's strongholds are close to Spiral Castle". Northward lay hills.
Historyenchanted sword Dyrnwyn to slay a shepherd who kept annoying him about righting a certain wrong. Rhitta kept the shepherd's property, banished his own war leader for reproaching him, and tyrannized his people, causing them to rebel.
Meanwhile the enchantment on his sword began to turn against him, and the scabbard grew black and blemished, preventing him from drawing it. Visited by the ghost of the shepherd he had slain, Rhitta forced Dyrnwyn out of its sheath against the blade's will. The sword's power struck Rhitta dead along with all his attendant warriors. The King was mourned only by the shepherd's ghost.Taran of Caer Dallben and Prince Gwydion were imprisoned there by Achren and her Cauldron-Born warriors. Eilonwy Daughter of Angharad helped Taran and Fflewddur Fflam escape, the latter by accident. While fleeing through the tunnels under Spiral Castle,
Gwydion later revealed it was the removal of Dyrnwyn -- a hard blow to the power of Arawn -- that caused the collapse.
Neither Gwydion nor Achren was in Spiral Castle when it collapsed, so neither of them died. Taran and the Companions mourned Gwydion at the heap of rubble, believing that the warrior had perished; later, Gwydion mistakenly did the same for Taran. All that remained of the once-proud castle was a single, square archway.