The evil fortress in Mordor in J.R.R. Tolkien’s works is Barad-dûr.
Barad-dûr, whose name means “Dark Tower,” is Sauron’s principal stronghold in Mordor. It stands near Mount Doom and serves as the centre of his power during the events of The Lord of the Rings. The One Ring must be destroyed in Mount Doom, close to the fortress.
Sauron first built Barad-dûr during the Second Age, using the power of the One Ring to make the structure possible. After Sauron’s defeat at the end of the Second Age, the fortress was destroyed, but it was rebuilt when Sauron returned to power in the Third Age.
Barad-dûr is often confused with Orthanc, the tower of Isengard, and Minas Morgul, the former Minas Ithil. Dol Guldur is another fortress associated with Sauron, but it lies in southern Mirkwood rather than Mordor.