The Perilous Gard by Elizabeth Marie Pope
This is a Newberry Honor Book from the 70's. I LOVED it! It was excellent. A fabulous blend of history and fantasy, The Perilous Gard is the tale of Kate Sutton, exiled from exile.
Which is not the double negative = a positive that you would think. It's house arrest at the home of a man she's never met.
And eventually, mysterious events lead her to the world of the Fairy Folk.
I quite liked Kate as a character. Her sister is an idiot, but what can you do? The last chapter, especially, is hilarious. (Not a good thing to be reading at midnight when everyone is asleep... you end up having to smother yourself.)