The Dark Lord's fortress in Mordor in J.R.R. Tolkien's works is Barad-dûr.
Barad-dûr is Sauron’s great fortress in Mordor. Its name is Sindarin for “Dark Tower,” and it stands near Mount Doom, where the One Ring was forged. The fortress is connected to Sauron’s power through the Ring, so its destruction follows the Ring’s destruction at the end of The Lord of the Rings.
Barad-dûr is not the same place as Minas Morgul, another major stronghold in Mordor, or Orthanc, the tower of Isengard. Minas Tirith is the principal city and fortress of Gondor, the kingdom opposing Sauron. In Tolkien’s narrative, Barad-dûr represents the military and symbolic centre of Sauron’s dominion. The tower is described as immense and dark, but much of its appearance is conveyed through the characters’ observations rather than through a complete architectural diagram.