Two-Moons and the Black Tower. Ardath Mayhar

Two-Moons-in-the-Sky, an Iroquoian warrior woman and Dreamer, is transported into the strange world of medieval Britain by a dark sorcerer, Lallius, who lives in a black tower. Lallius wants to enslave his captive, whom he regards as primitive, but little does he realize that Two-Moons has magic of her own! <P>
"A highly original fantasy with a strong and capable female protagonist"–Robert Reginald