There was a switch on the other side of the door. In order to leave that room with the second teleporter stone, you would need to have picked it up and used it (without dropping it), or picked it up after getting to the fourth level of the catacombs the long way.
Have you been kicked out of the castle yet? Thrown in a dungeon, and escaped? When you exited you got a quest to save Zandalor. Do so (treasure room sewer entrance) and let him teleport you somewhere. Clear that smallish area of monsters, and check out the room in the north west to get your teleporter stones back.
