Sauron’s fortress in Mordor in J.R.R. Tolkien’s works is Barad-dûr.
Barad-dûr is the Dark Lord’s principal stronghold during the Second and Third Ages. Its name is from Sindarin and is commonly translated as “Dark Fortress.” Sauron builds it in Mordor near Mount Doom, using the power invested in the One Ring to make the fortress exceptionally difficult to destroy.
The fortress falls when the One Ring is destroyed in the fires of Mount Doom. Because Sauron’s power is bound to the Ring, its destruction causes Barad-dûr to collapse. The event marks the end of the War of the Ring and Sauron’s defeat in the Third Age.
Barad-dûr is often confused with Minas Morgul, another major stronghold in Mordor. Minas Morgul is the corrupted former city of Minas Ithil, while Barad-dûr is Sauron’s own tower and seat of power.