Who is The Lady of the Ancient City from The Lais of Marie de France and what is their importance?

The Lady of the Ancient City appears in Guigemar. She is the wife of a jealous lord, the lady of the ancient city is kept locked away from the rest of the world. She is attended by her sympathetic niece and watched over by a feeble priest, so it is not difficult at first for her to conceal Guigemar in her quarters. Like many of the female characters in Marie's lays, she is trapped by a man she does not love.