I killed Lysandra then went up the stairs and killed several Undead Ronins. The problem is there is a door here that I can't open. I have backtracked nyself to look for any levers that I may have missed but can't find any. How do I get this door open?

Is this door on the far side of a rather dark room?? If yes - look in the room for a lever - just like those that you switch to enter a dungeon. I'm pretty sure this must be where you are - from Lysandra you'd be heading up to D'Soto's rooms.

Wyv