Okay, so, I've got the yellow torchkey and the purple torchkey, and I can't proceed. Do I really need to go to an early save and rebuild Lantry for Voices of Ages in order to advance the story? Or am I missing something?

Firstly, you definitely don't need Voice of Ages to advance the story. It's only used to read new magic runes (so IMO it's worth getting, but by no means essential to finishing the game).

There should be one other torchkey in those same oldwalls somewhere, though I can't remember what colour it is. The first time through I found that I missed one of those little "buttons" that appear when you use a torchkey. I'm pretty sure there's at least one case where you can use two different torchkeys on the same pedestal, each activating a different button.

So I'd suggest going through the whole level, checking for glowing buttons. At each torchkey pedestal (or whatever they're called), try using all torchkeys you've got at that point, then check to see if any new buttons have started glowing. Then of course push it to see what it opens.

From memory there's one button that is actually a fair distance from the actual torchkey pedestal - the screen is supposed to scroll to the spot where the button is when it's first revealed, but there's at least one I missed the first time through and it took me ages to find it (and lots of backtracking).

Also from memory, one of the torchkeys is looted from one of the bodies inside the oldwalls. The other is either from another body, or from one of those altar-like things (can't remember exact locations though).

Thanks! It actually turned out the level was bugged. In the room where the raiders were fighting the scourges, aside from the raiders attacking me, the beacon didn't work. So, I went through it again from an earlier save, and was able to open the door. Bah! Bugs.

I'll keep a lookout for that distant door, thanks.

Good to see that you managed to get through it. You might want to mention this on the official Tyranny forum so they can patch it though...

If you don't find that door in these Oldwalls, then it's in the Blade Grave Oldwalls - I keep getting these confused, although sometimes those glowing buttons are easily missed in either one. It doesn't help that there are some beacons in the Twin Rivers Oldwalls that can only be used with keys from the Blade Grave Oldwalls (and vice versa).