In J.R.R. Tolkien’s The Lord of the Rings, the giant fortress-like entrance to Mordor is called the Black Gate.
The Black Gate, also known as the Morannon, blocks the main northern entrance into Mordor. It is built between two mountain ranges, the Ephel Dúath and the Ered Lithui, and is heavily guarded by Sauron’s forces. The gate represents the most direct route into Mordor, but it is too strongly defended for Frodo and Sam to use safely.
Aragorn and the western army later march to the Black Gate as a diversion. Their purpose is to draw Sauron’s attention away from the real mission: carrying the One Ring to Mount Doom. Minas Morgul is another major stronghold in Mordor, while Barad-dûr is Sauron’s principal tower, so neither is the name of the entrance itself.