I have a perhaps related problem - I undertook to take on a quest for Nedmar before getting the key from him (as opposed to using threats/force to get it). My party then had to leave the area, either on to the battlements near the climbing vine entry point, or those giving access to the yard in the castle's centre. Every time I try to exit to the battlements, the game crashes (Steam, adequate PC set-up). Seems like a bug to me, any advice here before I "report an issue"? I went to bed at that point, insomnia has me up again, I'll explore again for alternative routes - none apparent.
Later: Okay, I found an internal staircase, attempted to use it, crashed. As far as I can see I'll have to go to an earlier saved game, before entering the castle, and resume. But I won't be able to complete this quest.