The Castle of Llyr (Prydain Chronicles Series #3)

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, Tara...