A secret entrance to the halls of Erebor, located on the western side of the Lonely Mountain.

It was through this door that Thror and Thrain II escaped when the dragon Smaug descended on the mountain top in TA 2770. Thorin Oakenshield was a child at the time, and already outdoors and so knew nothing of the Side-door until Gandalf gave him the map and key 170 years later.

It was in TA 2850, that Gandalf came upon Thráin II in the dungeons of Dol Guldur, close to death and unable to remember his own name. Gandalf received the map and key from him, but not knowing who he was or who his heirs might be it took him many more years to discover the truth. When he did, in TA 2941, he set in motion the Quest of Erebor , in which Thorin and Company enlisted Bilbo Baggins to help them raid Smaug's hoard.

The Side-door was accessible only by a narrow ledge that led to a grassy hollow in the mountainside. Set in a flat rock wall at the head of the hollow, the Side-door was invisible to the eye except under special circumstances. Runes on Thror's map noted its proportions as   "Five feet high the door and three may walk abreast." Elrond had also identified a further message in hidden runes or moon-letters. It told that the keyhole only became visible when the last sun-set rays of Durin's Day shone upon it.

The company had fortuitously arrived in the hollow on Durin's Day. It was moist, shady and verdant and home to many snails. It thus attracted thrushes, one of whom alerted them to the critical moment and thus the door was opened. From it a long low tunnel led down to the Lower Halls where Smaug lay upon his treasure.

Through this door, Bilbo Baggins entered the Lonely Mountain and later escaped, slightly singed. The tunnel and door were too small for Smaug to follow but were destroyed by him in his wrath.
Encyclopedia entry originally written by tarkena