The fortress where Saruman resides in Tolkien’s The Lord of the Rings is Isengard.
Isengard is a fortified ring of stone in the southern part of the Misty Mountains, near the river Isen. Saruman the White occupies it as the chief of the Istari in Middle-earth. The fortress includes the tower of Orthanc, where Saruman studies and later imprisons Gandalf.
Saruman transforms Isengard into a military-industrial base while preparing war against Rohan. His Orcs cut down trees, dig pits, and manufacture weapons there. This change reflects the destructive, mechanized character of his power and his growing imitation of Sauron.
The Ents attack Isengard during the War of the Ring. They breach its defenses and flood the surrounding area, leaving Orthanc standing in the middle of a lake. Saruman survives inside the tower for a time, but his authority and military strength are broken.
Isengard is separate from Barad-dûr, Sauron’s fortress in Mordor, and from Minas Tirith, the chief city of Gondor.