I forget the details of that particular room, but the tower does indeed have some secret tunnels. If you find yourself in that situation again, a friend can come and help -- get them to go on the quoin side of the wall and then follow.
Yep, this happened to me today too, as I was finally doing the tower quests for the first time today. As soon as you walk through the door to get to the other side of the wall, the quoin disappears...quite frustrating...but oh, well.