I'm at the end of the Glatzburg Episcopate Dungeon level 3, before the final door leading to the Cardinal Truncquillius (i.e. the door marked C4 on the map found on this forum). And I'm totally unable to open it.
I know there are for levers in a room to the west, but no matter how I try the door won't open. I've explored whole map of the level, I've seen all walkthroughs and discussions I could find, but still no luck. It seems I'm not the only one with this problem, but I've found no working solution (in fact, none at all).
I'm able to use a cheat to "summon" Truncquillius so it's possible to somehow continue with the quest, but I'd prefer going the "correct" way because I don't know if there is some important item behind that door and I fear summoning Truncquillius using a cheat may break the game somehow.

So, does anyone knows for sure how to open this door? Has someone had this problem too? Has anyone found a solution?

Solved!

SPOILER ALERT!





If someone has the same problem, know that those damned door is opened by a key which peacefully rests in a chest just before the previous door. The catch is that the key's name is the same as of some other key in that dungeon, so if you don't notice their visual difference you may leave it there because you think you already have it...