The star-shaped fortress built by Sauron in Mordor in Tolkien’s The Lord of the Rings is Barad-dûr.
Barad-dûr, whose name means “Dark Fortress,” stands in Mordor near Mount Doom. Sauron uses it as his principal stronghold and as the center of his power during the War of the Ring. The fortress is connected to the One Ring: when the Ring is destroyed in Mount Doom, Barad-dûr collapses.
Sauron begins constructing Barad-dûr during the Second Age, and the work takes centuries. The fortress is destroyed after Sauron’s first defeat at the end of that age, but he rebuilds it before the events of The Lord of the Rings. Its enormous scale and dark architecture make it a defining image of Mordor.
Barad-dûr is distinct from Orthanc, the tower of Saruman in Isengard, and Minas Tirith, the principal city of Gondor. Dol Guldur is another fortress associated with Sauron, located in southern Mirkwood rather than Mordor.