In the main hall of Silverlake castle where Lord Gawaan & the castellan reside after you rescue them, there's a curious door which is locked. Is it openable?

Hmm, are you talking about the archives? If not, can you post a screenshot?

No no. The archives' door i obtained the key for it. That's done. In the big hall with tables and the fireplace were the lord and the castellan reside. And sorry, i don't know how to take a screenshot :/

Ok, look in this video - https://www.youtube.com/watch?feature=player_detailpage&v=8YuM-RPIx0Y#t=966

Basically, you need to find a key in one of the towers.

As for screenshots - I recommend using Bandicam or Fraps. ;)

Oh man this can't be happening. Apart from the video you sent me ( many thanks ), i already saw other videos showing that BEFORE THE KEY, you need to find Lord Tronter's journal in a chest in one of the towers. Only when you find the journal, the quest is activated. I already opened the chest that shows the journal in it but i didn't find any journal. That's why i'm not findind the key in the place shown in the video. Could be a glitch? And i really REALLY have to open that door cause i'm collecting relics and there's one inside :/

There is no glitch. I solved it myself cause i missed an important thing. Now ok :)) But thanks anyway mate.