#barad dur

Therefore [Sauron’s] flight was but a feint, and he soon returned, and ere the Wise could prevent him he re-entered his kingdom in Mordor and reared once again the dark towers of Barad-dûr. ~ The Silmarillion, Of the Rings of Power (Art: “Approaching Barad-dur” by flatcapart on DeviantART)

A strong place and wonderful was Isengard, and long it had been beautiful; and there great lords had dwelt, the wardens of Gondor upon the West, and wise men that watched the stars. But Saruman had slowly shaped it to his shifting purposes, and made it better, as he thought, being deceived–for all those arts and subtle devises for which he forsook his former wisdom, and which fondly he imagined were his own, came but from Mordor; so that what he made was naught, only a little copy, a child’s model or a slave’s flattery, of that vast fortress, armoury, prison, furnace of great power, Barad-dûr, the Dark Tower, which suffered no rival, and laughed at flattery, biding its time, secure in its pride and its immeasurable strength.

–J.R.R. Tolkien, The Two Towers, “The Road to Isengard”

Barad Dur, the Dark Tower, was built by Sauron. Its construction started in !000 Second Age and took 600 years to be completed. It was the greatest fortress to ever exist in Middle Earth, apart from Angband. It was razed after the War of the Last Alliance, and rebuilt after Sauron came back to Mordor. It was here that Gollum was tortured. It also has pits and caverns underneath it, that probably serve as prisons or dwellings. IT was destroyed with the destruction of the One Ring, like Sauron.