Back-door

The Back-door also called the Goblin-gate is a stone door on the eastern side of the Misty Mountains.

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.

History
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.

Significance in The Hobbit
Pursued by Gollum and a group of orcs, the invisible Bilbo barely manages to escape through the door loosing a few buttons on his waist coat in the process.