The Golden Horn (The Hound and the Falcon Trilogy, Vol 2)

Constantinople, A.D. 1203: a great army camps outside the City of Cities, and threatens it with conquest. Within the City, a Greek witch and a renegade monk who is also a white enchanter do their utmost to hold back the tides of war. But even their great powers may not be able to save Byzantium's empire from the Western Crusade. Alfred, a healer with special powers, and Thea, a shapechanger, are captured when Crusaders capture the city of Constantinople