The Perilous Gard by Elizabeth Marie Pope

The Perilous Gard is about a girl who's been sent away to a castle that's full of all kinds of mysterious happenings: disappearing heiresses, pilgrims trekking to a wishing well, and sinister strangers lurking about in the forest.

Kate doesn't really believe the superstitious stories she hears about the "fairy folk" who live near Elvenwood until she finds herself a captive of the People of the Hill - an ancient, mystical tribe who are still practicing the old Celtic religion during the Elizabethan age.

I can't say I'm usually a big fan of this type of thing, but Kate's just the kind of intelligent heroine I most enjoy reading about, and her love interest is just as compelling. It's worth a look, even if you don't normally like fantasy.