Angband

Angband, also known as the Iron Prison, was the ancient fortress of Melkor in the depths of the Iron Mountains that was built after the destruction of the Two Lamps as an arsenal of Utumno, but later it became the Dark Lord's primary stronghold from which he relentlessly sought to dominate Arda. Angband was finally destroyed by the Host of the West at the conclusion of the War of Wrath.

Angband was a primarily subterranean stronghold under the three volcanic mountains of Thangorodrim, the largest mountains in Middle-earth. Before the Great Gate, there was a somber court area flanked by frightening cliffs and walled by the towers of a great battlement. Through the gate, there was a long great tunnel leading to a 'labyrinthine pyramid' of stairs to corridors, tunnels, and smithy chambers. A tall chimney went from Morgoth's gigantic blast furnaces and smithies, up through the mountain to the smokey towers of Thangorodrim. There were also many tunnels leading to the slave quarters or vaults. At the foundation of Angband was the entrance to Morgoth's throne room in the Nethermost Hall. There were also mines within Angband that extended deeper than Morgoth's throne room.

History
Melkor originally built Angband to guard against a possible attack from Aman by the Valar, and placed it under the command of Sauron. However, at the initiation of the War for Sake of the Elves, the assault of the Valar leveled Angband without much difficulty, and they swiftly moved on to Melkor's primary stronghold of Utumno. However, Angband's underground vaults and caverns were left relatively intact after the Valar's onslaught, as they were in great haste to capture Melkor for the sake of keeping the newly-awakened Elves safe. After Melkor's defeat at Utumno, the Balrogs came to Angband and went into hibernation in its ruined foundations. Eventually, the Orcs began to multiply in great numbers in Angband's ruins, and soon made their way south into Beleriand, threatening the Dwarves and Thingol's kingdom. It is possible that Sauron remained in the ruins of Angband with the Balrogs, as he had presumably been commanding the fortress when it was attacked. However, it is not certain where Sauron dwelt during Melkor's imprisonment. After three ages, Melkor was released, destroyed the Two Trees of Valinor, stole the Silmarils and escaped from Aman.

 
After fleeing, Melkor, now named Morgoth by his enemies, returned to Middle-earth and re-delved the ruins of Angband, raising the volcanic peaks of Thangorodrim over it. These peaks were created from the slag and rubble caused by the re-delving of the fortress, and as such, it is likely that the rebuilt Angband extended for a considerably greater distance underground than its original incarnation. From his dark throne in the Nethermost Hall, Morgoth directed the war against the Ñoldor who had returned from the West, and all the Free Peoples that supported them in Beleriand. Ultimately, he achieved complete victory over them. At the end of the First Age, the Valar took pity on the Ñoldor and instigated the War of Wrath, which not only utterly destroyed Angband but broke most of northern Middle-earth so thoroughly that it sank beneath the ocean.