From a High Tower (Elemental Masters, #10)

From a High Tower is the newest adventure in Mercedes Lackey's Elemental Masters series.
When a man is caught stealing from a walled garden owned by a strange woman, he bargains away his youngest daughter in return for food for his family. The woman, rumored to be a witch, takes the golden-haired child and locks her away in a high tower. Sixteen years later, Giselle has lived an isolated life, but her adoptive mother has trained her in Air magic, and Giselle must use her new skills to keep herself and her new friends safe...