Cynthia and Constantine [Beyond Camelot: Brother Knights Book 1] by Kathye Quick
Lady Cynthia of Abertaine is trapped. Not only has her fiancé, Sir William Leyborne, not been back to the castle for over ten years, but she's also not a titled Lady. Lord Simon of Cowell, a renegade warlord aligned with Mordred against Arthur and his Knights, has declared himself sovereign over Leybourne Castle and everything that once belonged to Sir William--including Cynthia. Sir Constantine, Knight of the Round Table, has come to the shire to give Cynthia the news that her fiancé has fallen in battle. With him is William's oral will giving all he owns to Cynthia as though they had been wed. But when he finds Cynthia and discovers the shire under the control of an evil warlord, he knows he cannot leave without first driving Simon and his soldiers from the land. Drawn together by an attraction older than time, Cynthia and Constantine soon discover that though a vow made by a knight's honor has brought them together, it may just also cost them their lives.