Sauron’s fortress in J.R.R. Tolkien’s The Lord of the Rings is called Barad-dûr.
Barad-dûr is the Dark Lord’s great stronghold in Mordor, near Mount Doom. Tolkien describes it as a vast fortified tower and the central seat of Sauron’s power during the War of the Ring.
The fortress was originally built in the Second Age and was destroyed after Sauron’s defeat at the end of that age. Sauron later rebuilt it, but the One Ring’s destruction caused the foundations of his power to fail and Barad-dûr to collapse.
Readers sometimes mix it up with Isengard, Saruman’s fortress in the west, or Minas Morgul, the former Minas Ithil. Dol Guldur is another Sauron-associated stronghold, located in southern Mirkwood.