The Tower at Stony Wood - Patricia A. Mckillip

During wedding celebrations for his king, Cyan Dag, a knight of Gloinmere, is told by a mysterious bard the king has married a false queen--a lie cloaked in ancient and powerful sorcery. Cyan departs on a quest to rescue the real queen, to prevent a war, and awaken magic in a land that has lost its way. She saw the knight in the mirror at sunset... During the wedding festivities of his king, Cyan Dag, a knight of Gloinmere, is sought out by a mysterious bard and told a terrifying tale: that the king has married a false queen--a lie cloaked in ancient and powerful sorcery. Spurred on by his steadfast honor and loyalty, Cyan departs on a dangerous quest to rescue the real queen from her tower prison, to prevent war, and to awaken magic in a land that has lost its way...