After killing xydusa there is only one character alive. Is there any way
to open the door at the opposite side of the back entrance ?

No - the only way out is to continue through the whole area and it can be just as tough. You would probably be better off restoring your game and trying your battle with Xydussa again. How did you fight her? The strategy you use to battle her can make a huge difference in the outcome. Hope you saved your game before you got to her.

Wyv