[Firestormalpha]

I may know where you are stuck. There is a room north of the "Throne Room" that looks like a torture chamber of some sort. Beyond that past a few traps that can't be disabled (which would severely tick me off if it was BG2) there is a room that appears to be a dead end. There is a container in the center of the wall in the middle of the room which holds a ward stone. When you pick up the ward stone a glyph on the ground glows. When you move your mouse pointer over the glyph it turns into the wagon wheel thing (you know says your gonna move from one area to the next. Click that and it takes you to the other side of the wall. Follow that path and you will evntually end up at the room with the stairs that lead nowhere. In the south of that room there is a secret door that was previously locked. I don't know the exact conditions met to unlock it but you should have acquired a new ward or special key maybe triggered a switch down the other path.