The Side-door or Secret Door was a secret entrance to the Dwarven kingdom of the Lonely Mountain (Erebor) located on the western side of the mountain, from a shelf on the outside. It opened on to a long tunnel that led down to the Bottommost Cellar.
The Side-door was made as if to look like a rock face on the mountain and didn't look like a natural rock surface and all indications were that it was constructed thing. Instead, its surface was flat and smooth and there was no signs that it was a door at all. The door was five feet high with a keyhole about three feet from the ground.
The Side-door was created sometime during the reign of Thrór with the support of his son Thráin. Their reason for creating it was unknown but literature points toward a means of escape. According to Thorin, Thrór and Thráin were the only ones who knew about it and used it to escape when Smaug attacked and claimed the mountain and all its wealth. The door could only be opened with a key, and then only on the last light shown upon it on Durin's Day. When Thorin and Company reached it, they recognized it for what it was and the dwarves eager to get inside attempted to force it open without success.
As a ray of sun struck the door, a piece of rock would flake off to reveal the key-hole, allowing the door to be opened. Bilbo passed through the door and down to Smaug. At the anticipation that Smaug would find out how he got in, the dwarves entered the tunnel and shut the door then Smaug attacked the entrance smashing the rock face to bits, sealing them in the mountain. The Side-door was most likely heavily damaged or destroyed by the angry dragon.
- ↑ 1.0 1.1 The Hobbit: On The Doorstep
- ↑ The Hobbit: An Unexpected Party
- ↑ 3.0 3.1 The Hobbit: Inside Information