"Maybe the doorway on the other side? The one with the huge portcullis?"
Sorry, I forgot to snap a picture of the full map last night. I reminded myself to do so while on my commute home, and then it just slipped me mind entirely. However, TomB is totally correct here. Good job."There is, if you go west from the "T" shaped corridor into the room at the end. There's a door in the north wall."