The Castle of Llyr by Lloyd Alexander, Book Three in The Chronicles of
Prydain
Princess Eilonwy hates to leave her friend Taran, Assistant Pig-Keeper,
and her beloved home, Caer Dallben. Why does she have to go to the Isle
of Mona to train as a proper lady when she's already a princess? But
Eilonwy soon faces much more than the ordeal of becoming a dignified
young maiden, for she possesses magical powers sought by the evil
enchantress Queen Achren.
When Eilonwy is put under a deep spell, Taran and his companions set out
on a dangerous quest to rescue her. Yet how can a lowly Assistant
Pig-Keeper hope to stand against the most evil enchantress in all of
Prydain?