The Perilous Gard, by Elizabeth Marie Pope, 1974

When rational-minded Kate Sutton is unjustly banished to Sir Geoffrey Heron’s remote northern castle by Mary, Queen of Scots, she finds that old heathen myths and ballads are more than superstition. Kate’s determination unearths a nightmarish ancient tradition that entraps Sir Geoffrey’s daughter and younger brother. A satisfying foray into history as well as the subterranean world of the fairy-folk, complete with mystery, danger, ritual, and well-drawn romance. Kate is a convincing heroine whose weaknesses and strengths equip her to withstand a magic both mythic and psychological. – Julianna Berry