In J.R.R. Tolkien’s legendarium, the fortress ruled by Saruman is Isengard.
Isengard is a fortified ring in the southern part of the Misty Mountains, near the Gap of Rohan. At its center stands Orthanc, a tall, dark tower built by the Dúnedain. Saruman establishes his seat there after becoming head of the White Council and later secretly serving his own ambitions.
Saruman uses Isengard as a military and industrial base. He breeds the Uruk-hai, gathers weapons, and damages the surrounding landscape. His activities make the fortress a major threat to Rohan and to the peoples opposing Sauron.
Isengard and Orthanc are closely connected but are not identical names. Isengard refers to the fortified domain, while Orthanc is the central tower. The Ents attack Isengard during the War of the Ring, flood much of it, and imprison Saruman in Orthanc before later allowing him to depart.