The Tower at Stony Wood

Cyan Dag, knight of Gloinmere, is sworn to serve King Regis Aurum of Yves. Cyan's oath leads him headlong into dangerous magical territory, however, when Idra, Bard of Skye, reveals that the King's new bride, Lady Gwynne, is an impostor. The true Lady Gwynne is trapped in an enchanted stone tower in distant Skye, a magical mirror her only means of viewing the outside world. Bound by his oath to protect the King, Cyan rides west to free Lady Gwynne. In the meantime, Thayne Ysse, son of the king of Ysse, has never forgotten his father's defeat at the hands of King Regis Aurum. Now he seeks a tower guarded by a dragon, a tower filled with gold enough to raise a new army and defeat Yves once and for all. And in another ancient tower outside the coastal village of Stony Wood, Melanthos, the daughter of a land-bound selkie and a fisherman, obsessively embroiders pictures of a lonely woman trapped in a distant tower who may or may not be real. Although Cyan Dag took up his quest with one goal in mind, he soon realizes that the only route to saving Lady Gwynne lies tangled with the lives of Thayne and Melanthos, and in the mysterious motives of Idra and her woods-wise sister Sidera.

Synopsis from Open Library.