
"Isle of Mona" is a version of Ynys Môn, the Welsh name for the Isle of Anglesey. Nearly all of the proper names in Prydain are historical or mythological. 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. During the adventure, more is revealed of Eilonwy's heritage and her family's former home, the castle Caer Colur. Soon after her arrival, she is kidnapped by agents of the evil sorceress Achren, who wishes to use her for as part of a plan for domination. Princess Eilonwy, the latest enchantress to be born of the House of Llyr, "faces the unavoidable (and in her view absolutely unnecessary) ordeal of becoming a young lady." The young hero Taran accompanies her on her journey to the royal court of the Isle of Mona, the same island where her ancestral home was located, and where she will continue her education as a princess.

The story continues the adventures of Taran, the "Assistant Pig-Keeper", and his companions. The Castle of Llyr (1966) is a high fantasy novel by Lloyd Alexander, the third of five volumes in The Chronicles of Prydain.
