here lives George p by Philip Reeve

Welcome to the dark side of Camelot. Gwynna is just a girl who is forced to run when her village is attacked and burns to the ground. To her horror, she is discovered in the wood. But it is Myrddin the bard who has found her, a traveler and spinner of tales. He agrees to protect Gwynna if she will agree to be bound in service to him. Gwynna is frightened but intrigued-and says yes-for this Myrddin serves the young, rough, and powerful Arthur. In the course of their travels, Myrddin transforms Gwynna into the mysterious Lady of the Lake, a boy warrior, and a spy. It is part of a plot to transform Arthur from the leader of (con't)