The Castle of Llyr

The series was inspired by Welsh mythology and by the castles, scenery, and language of Wales, which the author experienced during World War II army combat intelligence training.[2][3]

All of the proper names in Prydain are historical or mythological.[2] "Isle of Mona" is a version of Ynys Môn, the Welsh name for the Isle of Anglesey, but that island is very close to the northwest point of mainland Wales, crossed by two highway bridges. In the author's words, the Prydain chronicles communicate "the feeling, not the fact, of the land of Wales and its legends".[4]

The story begins 18 months after the destruction of the Black Cauldron. Eilonwy is the last in a line of royal sorceresses of the House of Llyr, and Dallben the enchanter has decided that Eilonwy, as a princess, needs an education that he cannot provide. He sends her to reside at a royal court on the Isle of Mona, in the west of Prydain. Taran and Gurgi escort her to Mona, on a ship "captained" by Prince Rhun, a cheerful but incompetent youth. Taran is finally aware of his feelings for Eilonwy and he envies the prince's noble birth.

While Eilonwy is introduced to the tedium of life at court, Taran encounters his frequent companion Fflewddur Fflam—a minor king who lives as a wandering bard—and a shoemaker, who turns out to be none other than Prince Gwydion. Gwydion tells Taran that Eilonwy is in grave danger, very likely from the evil sorceress Achren, whom Taran and Eilonwy escaped in The Book of Three. He charges Taran with helping to protect Eilonwy.

Soon after, Taran and Gwydion witness Chief Steward Magg leave the castle at night to signal a ship at sea. When Magg and Eilonwy do not show for breakfast the next morning, King Rhuddlum organizes search parties. Taran is assigned to the party nominally under the command of Prince Rhun, and the king asks Taran to protect Rhun personally. He confides that he and Queen Teleria hope to betroth Rhun to Eilonwy. Although he is appalled and envious, Taran vows to ensure Rhun's safety.

Shortly before dusk, the prince separates from the group. Taran pursues him, joined by Fflewddur and Gurgi; the next morning they find him at an abandoned hut in the woods. Inside they find a small book of blank pages that Rhun decides to keep for himself, and a sheaf of notes by the former resident, Glew. Evidently Glew was a careful experimenter with potions, and he developed one that successfully enlarged a young mountain cat he named Llyan. As they prepare to leave, Llyan herself returns—now larger than a horse—and settles down just inside the door, seemingly intending to eat the companions. She is entranced by Fflewddur's harp playing, which allows the companions to escape.

Later that day, Taran's pet crow Kaw spots Magg and Eilonwy heading for the river Alaw on horseback, and Taran determines to pursue them. By the river, Rhun finds Eilonwy's golden bauble—the Golden Pelydryn—near tracks indicating that Magg and Eilonwy dismounted and fetched a boat. The companions build a hasty raft to follow downstream, but it disintegrates before reaching the mouth. While repairing the raft, Rhun manages to tumble into a deep pit, and his attempted rescue prompts a landslide that traps all four. They explore the caverns by the light of Eilonwy's bauble and eventually find Glew, who is trapped by his giant size. The companions promise him Dallben's aid and persuade him to show them a way out, but he leads them to a dead-end tunnel and traps them. Sobbing pitifully, he explains that he knows the recipe for a potion that will diminish him, but he must kill one of them for a final ingredient, and will then free the others.

Rhun surprises everyone by volunteering, for he has recognized that he is a burden to all and incompetent to rule. However, before Glew returns they notice an exit above their heads and construct a human ladder which enables Rhun to escape the cavern, promising to return to the city and bring help. When Glew opens the cave to retrieve his final ingredient, the trio breaks out and attacks him. Meanwhile, Rhun has not left the caverns but returned by another route with the help of the Golden Pelydryn. The blazing light of the bauble blinds the giant, allowing the companions to escape. As they leave the cavern, Taran discovers that Rhun's book of blank pages actually contains writing none of them can read, visible only under the light of the bauble.

When they reach the mouth of Alaw on the reconstructed raft, they meet Gwydion, who hides them and tells what he knows. He has visited the offshore ruin of Caer Colur, the ancestral home of the house of Llyr, and seen Achren, Magg, and Eilonwy with several mercenary guards. Gwydion reveals that the book of blank pages contains the House of Llyr's most powerful enchantments. Achren hopes to rule by bringing Eilonwy to her full ancestral powers yet maintaining control by her personal magic. At night Gwydion rows them all to land below the seaward walls and hides the book and bauble before they begin to search.

Taran climbs to the tower room where Eilonwy resides, only to find that she does not recognize him or any of the names of her former companions. She flees from her room and cries out. Taran vainly hopes to halt the alarm and follows her until Magg arrests him.

Gwydion, Fflewddur, and Gurgi have entered the halls as well, and they have the better of a struggle with Magg and some guards, until Eilonwy and Achren appear, with the princess entirely under the witch's control. Rhun stupidly reveals that they know the location of the book and Pelydryn, which Achren needs to complete her control of the House of Llyr's magic. Achren turns to Taran and offers to pay him for what he knows: she will restore Eilonwy's memories of him and allow them to wed. Gwydion interrupts the torment by revealing the cache.

When Eilonwy takes hold of the heirlooms and begins to examine the book in the light of the bauble, she also begins to fight against Achren's spell and finally makes her own choice. Calling upon the full power of the Pelydryn, she incinerates the book in a column of crimson flame. Meanwhile, Magg has responded to Achren's scorn by opening the gates that protect the castle from the sea, and he and the surviving guard have escaped by the only ship.

As the castle floods, Taran loses consciousness; he awakes to discover that all reached the shallows alive, and Llyan pulled them up the beach. Eilonwy tells how she was kidnapped by Magg and lost her bauble en route to Caer Colur. Before leaving the sea, she finds a ceremonial horn that has washed ashore, which she calls "all that's left of Caer Colur". She gives it to Taran as a pledge that she will not forget him during her tenure at Dinas Rhydnant. He can pledge only his word in return, but "the word of an Assistant Pig-Keeper ... shall do very well indeed." He mentions the royal plan to engage her and Prince Rhun, which she scorns him for taking seriously.

Caer Colur was the seat of power of the House of Llyr, where Eilonwy's grandmother Queen Regat was the last in the line of women to reign. It stood at the northeast point of the Isle of Mona, just north of the mouth of River Alaw, some ways by ship west of northwest Prydain (a region never visited in the Chronicles).

The point of land broke and began to sink with the castle, which was abandoned after Princess Angharad eloped with "the True Enchanter" Geraint against her mother's orders. She traveled with the book of spells and the Golden Pelydryn necessary to read them. But countless magical implements remained at Caer Colur.

American Library Association: Notable Children's Book

• Tuck, Donald H. (1974). The Encyclopedia of Science Fiction and Fantasy. Chicago: Advent. p. 6.