I'm going through the various treasure hunts, and in the process of finding the Bear School gear, I got into the throne room of Clan Tuirseach's castle, got the stuff I came for, but now there doesn't seem to be any way to get out. The entrance via the dungeons closed behind me, and the exit appears to be a gate exiting the throne room, but pulling the lever next to the gate doesn't do anything. I've been stuck here for 45 minutes now, and other than the lever next to the door (again; doesn't do anything), I can't even find anything else that interactable in the area, with the exception of torches.

Yep, it's a rather irksome bug...
There's another thread about the same topic already, I recommend going with this user's solution (worked fine for me ;) ).

Edit for clarification: From what I remember/have seen in walkthroughs that lever in the throne room is supposed to open the gate, but apparently doesn't work for a number of people...

well, after following that video, I did manage to get out, however, I needed to get back into the castle so I could grab some chests I'd bypassed in the dungeon area. (I'd reloaded from a previous save game, then tried running through without grabbing anything to see if it would make a difference. It didn't.)

After opening the chests and getting the epic poem as well as the Enhanced Black Blood potion, I went to pull the lever to open the dungeons again, and it no longer worked. Apparently, once you open the cells the first time, that's it, and the lever stops working.

The staircase going back up to the main floor where I entered from doesn't work either, as there's a sheer drop of several steps halfway down it, and it's impossible (or at least really really hard) to climb back up.

So now I'm stuck in a completely different area OF THE SAME CASTLE.

*sigh*

I guess it's back to the old save game again.