The Marsh King's Daughter.

1999 Little Brown hardcover, Elizabeth Chadwick (The Autumn Throne: A Novel of Eleanor of Aquitaine). Unwanted and unloved, Miriel Weaver finds herself forced into a closed convent by her violent stepfather. Outside, the civil war of 1216 rages through the English countryside, throwing into jeopardy all that its people hold dear. As the turmoil outside reaches a peak, Miriel itches to break free from her life inside as a religious novice. She plots to escape but her plans screech to a halt when a soldier of fortune, a half-dead Nicholas de Caen appears at the convent door. - Goodreads