The Back-door (also called the Goblin-gate) was a stone door on the eastern side of the Misty Mountains. It was used as a means of escape in the event of an emergency, and also as access to the lands beyond.
It could be reached by the tunnel that came from the Great Goblin's cavern and the one that came up from Gollum's cave. The room that it was located in was somewhat small in size.
The door was evidently built by the orcs when they tunneled out what was later called Goblin-town deep within the Misty Mountains, during their great increases in number between the years TA 2060 and TA 2480.