The evil fortress ruled by Sauron in J.R.R. Tolkien’s Mordor is Barad-dûr.
Barad-dûr, whose name means “Dark Fortress,” is Sauron’s principal stronghold in Mordor. It stands near Mount Doom and is linked to Sauron’s power through the One Ring. The fortress is rebuilt during the Second Age after Sauron establishes himself in Mordor and becomes the center of his military power.
Barad-dûr is destroyed when the One Ring is cast into the fires of Mount Doom. Its fall follows Sauron’s defeat at the end of the Third Age. Isengard is Saruman’s fortress in the west, Minas Morgul is the corrupted city overlooking Cirith Ungol, and Dol Guldur is a separate stronghold in southern Mirkwood.