The Horned King's Castle, also once known as the Ancient Castle, was a decrepit and sinister palace home to the Horned King himself.
The history of this ancient bastion found in mountainous region within the land of Prydain is shrouded in mystery, but according to Princess Eilonwy, this was a magnificent castle once home to a great king who built it before the Horned King killed him and easily took it over as his own fortress many years ago.
The Horned King ruled from his castle with an iron first as he commands a army of human brutes and a legion of Gwythaints to obey him and execute his enemies.
Taran traveled to and sneak in to this eerie castle in a mountainous region to rescue the clairvoyant pig Hen Wen who later flees from the Horned King's court and escaped but Taran is captured himself and thrown into the dungeon, soon to be released by Princess Eilonwy, a girl of his age who is also trying to escape. In the catacombs beneath the castle, Taran and Eilonwy discover the ancient burial chamber of a king, where he arms himself with the king's sword. It contains magic that allows him effectively to fight the Horned King's minions and so to fulfill his dream of heroism. Along with a third captive, the comical, middle-aged bard Fflewddur Fflam, they escape the castle and are soon reunited with Gurgi.
Creeper, the Horned King's royal goblin henchman, cowardly to reports the recent escape of the prisoners to the King in his throne room. The King than ordered him to send out the Gwythaints and followed them so they can lead him to acquire the Black Cauldron he desires.
Later, the Horned King's mortal minions succeeded in capturing Taran and his friends except Gurgi and recovered the Black Cauldron as the 3 heroes were chained in a sacrificial chamber within his castle. The evil king then begins a dark ceremony and summons the Cauldron Born. The undead soldiers came to life and gruesomely attacked the useless human henchmen that the Horned King no longer needed that he has the invincible army at his command. Soon after that before the Horned King's plans of total dominion over Prydain can be complete, Gurgi sacrificed himself by jumping into the Cauldron, causing the artifact's power to reverse and even drain the life force of the Cauldron Born. After the death of the Horned King, who tried to kill Taran but was destroyed by the spirit of another tyrannical king encased in the Black Cauldron, the Cauldron itself sink into the earth while its magical powers caused the entire castle to crumble and erupt in flames. Taran and his friends escaped the castle with their lives.
- The Horned King's castle is very similar to Maleficent's castle in Sleeping Beauty.
- In the original Chronicles of Prydain, Taran is locked up in Spiral Castle, where he meets Eilonwy and Fflewdurr. The owner of the castle and the previous owner of the Magic Sword Dyrnwyn was King Rhitta, who went insane with the guilt of killing an innocent with the sword that it eventually killed him in his makeshift tomb. It later served as as the headquarters of the enchantress Achren, until Taran removed the sword from the tomb, leading to its collapse.