In The Lord of the Rings: The Return of the King, the giant fortress ruled by Sauron is Barad-dûr.
Barad-dûr stands in Mordor near Mount Doom and functions as Sauron’s principal stronghold. Its towering form dominates the landscape and symbolizes the Dark Lord’s power.
The armies of Mordor gather around the fortress during the final stages of the story. The quest succeeds only when the One Ring is destroyed in the fires of Mount Doom, causing Sauron’s power and fortress to collapse.
Orthanc is Saruman’s tower at Isengard, while Minas Morgul is a different city in Mordor. Dol Guldur is another fortress associated with Sauron, but it is not Barad-dûr.