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Lonely Mountain, or Erebor, was a mountain in the north-east of Rhovanion. It was the source of the river Running, and a major Dwarven stronghold, the Kingdom under the Mountain at the end of the Third Age and well into the Fourth.
With the awakening of Durin's Bane in the capital of Khazad-dûm, Thráin I led some Dwarves to Erebor. They dug its caves and halls forming an underground city, and established the Kingdom under the Mountain in T.A. 1999. During Thráin's rule many riches were mined from its depths, like the Arkenstone. The town of Dale was also built by Men between its slopes.
Thráin's son Thorin I abandoned the Mountain in T.A. 2210 for the Grey Mountains; but after the War of the Dwarves and Dragons, in T.A. 2590, King Thrór led a group back to the Lonely Mountain, re-establishing it as the capital of Durin's folk.
The great dragon Smaug had lust for the Dwarven riches and in T.A. 2770 he descended on the mountain driving out the Dwarves and destroying the town of Dale. The Lonely Mountain was empty for almost two hundred years, save Smaug who slept in the innermost chamber on a great pile of wealth.
In the year T.A. 2941 with Gandalf's council, king Thorin II and a small company of friends and family actually made it to the Lonely Mountain. After the Dragon Smaug had realized that the Dwarves had been helped by the Lake-men he went to their town of Esgaroth intent on destroying them, only to be killed by a man named Bard.
 Return of the Longbeards
With the help of a Hobbit named Bilbo Baggins Thorin and company were able to retake the city and the treasure, therefore allowing Thorin to proclaim himself King under the Mountain. But after refusing to give any of the treasure to the Men of Esgaroth, and the Elves of Mirkwood, the mountain and the Dwarves and Hobbit in it were put under a bloodless siege.
Things nearly came to blows when Thorin's cousin Dáin Ironfoot (Grór's grandson) arrived as aid to his kinsman and nearly went to battle against the besiegers. But Gandalf interceded and warned them all of a great host of Orcs and Wargs coming to take the mountain. So the Elves, Men, and Dwarves made an alliance, and fought a bloody battle against their foes in the valley before the gate. In the end the defenders were victorious against the Orcs and Wargs. Thorin was mortally wounded during the battle, but finally, after so many years of longing, Dáin returned the Longbeards to the Lonely Mountain.
Many years later, during the War of the Ring, conflict broke out between Dale and the East and eventually Men and Dwarves retreated into the mountain. King Brand and King Dáin were killed at its very gates. The Dwarves and Men held out for several days until word reached the ears of the Easterlings that the great hosts of Sauron in the south had been defeated in the Battle of the Morannon. With this news fear fell on the besiegers. When the besieged saw this they came forth from the Lonely Mountain, and attacked their enemy driving them from Dale across the Running.
Erebor and Dale continued to prosper into the Fourth Age.
The Lonely Mountain was possibly 3,500 feet tall, as it was snowcapped at spring. Geologically, it was rich in metals and jewels.
The mountain was star-shaped with six ridges radiating as spurs from the peak.
The south-western spur contained Ravenhill housing a Dwarven guard-post. Between the two western spurs was a narrow vale which was the exit of the Back Door, behind an overhanging cliff. Rough steps ascended to the top of the southern ridge along a narrow ledge turning east behind a boulder into a steep bay.
Inside, the mountain was dug with passages and tunnels leading to cellars and halls and mansions such as the great chamber of Thrór near the Front Gate. A secret tunnel led to the "bottommost cellar"
The main entrance into the mountain was the Gate of Erebor on the south side, opening onto a valley between two great spurs of the mountain. At the end of the southwestern spur was Ravenhill, where there was a lookout post. The River Running sprang from beneath the mountain and issued from the Front Gate, forming a waterfall that fell into the valley below.
Inside the gate was a broad paved road that went alongside the river in a wide curve leading into the mountain. Not very far from the entrance was the Great Chamber of Thrór where feasts and councils were held.
In the Lower Halls, there was a vast chamber called the Great Hall of Thráin at the root of the mountain. From there a secret passageway led to a hidden door in the western side of the mountain. The Back Door was invisible from the outside except on Durin's Day, when the light of the setting sun would reveal the keyhole.
- J.R.R. Tolkien, The Hobbit, passim.
- J.R.R. Tolkien, The Fellowship of the Ring, Many Meetings
- J.R.R. Tolkien, The Fellowship of the Ring, The Council of Elrond
- J.R.R. Tolkien, The Return of the King, passim.
- J.R.R. Tolkien, The Lord of the Rings, Appendix A: III Durin's Folk
- J.R.R. Tolkien and Christopher Tolkien (ed.), Unfinished Tales, The Quest of Erebor
- J.R.R. Tolkien and Christopher Tolkien (ed.), The Peoples of Middle-earth