Most of you have obviously past this point which makes this doubly frustrating! Just where is the Circle within the Library (indeed where is the library?)? I've been to see D'Soto in the Tower, and explored everywhere. I have not been able to open the door at the back of the room containing the demon. Is it behind that door?

The demon is in there, and you don't want to go in until the very last. I'm not sure exactly what you mean by 'the circle in the library' ... tell me more.

Wait, my memory is returning. You go up a long spiral staircase to get to the library; the circle is obvious. As I recall, there is a large cathedral type room where you have a fight with many monsters. Through a door off that room is a room where you meet an important npc. In that room a large spiral staircase.

If that's not it, it's not particularly hidden. I came across it in routine exploring before I knew what it was for. You need to complete that ceremony before you face the demon. Sorry I can't be more specific.

OK you are looking for the alter to place the idols on I guess. You enter and area with a stirway going up I seem to remember 2 doors when you enter but the other way in is through the one way wall from the sewers. At the top of the stairs there is a gate on your right you cannot open and further on a room were the guy who gave you the idol in the woods obviously lives. Use an object (I think it was his relic) on the lock a secrect passageway opens. This ends in a room with a lever. Pull the lever which opens the gate. Go back to the gate this leads to the stairs up to the library and alter.