The dark tower in Mordor in J.R.R. Tolkien’s The Lord of the Rings is Barad-dûr.
Barad-dûr is the fortress and principal stronghold of Sauron. Its name means “Dark Fortress” in Tolkien’s constructed language Sindarin. The tower stands near Mount Doom and dominates the land of Mordor as a symbol of Sauron’s power.
The One Ring’s destruction causes Barad-dûr to collapse. Its fall marks the physical and political defeat of Sauron at the end of the story.
Orthanc is a different tower, located in Isengard and occupied by Saruman. Minas Morgul is another major fortress in Mordor, formerly called Minas Ithil. These similarly ominous places are often confused, but Barad-dûr is specifically Sauron’s Dark Tower.