The tower where Saruman dwells in Isengard is Orthanc. It stands at the centre of the circular fortress of Isengard, in the Wizard’s Vale beside the river Isen. Saruman occupies the tower after Gondor entrusts him with its keeping, eventually turning the surrounding valley into a militarized industrial complex.
Orthanc was built by the Men of Gondor toward the end of the Second Age from four great columns of rock joined and hardened by an unknown process. It rises more than 500 feet above the plain and ends in four sharp peaks. The tower also houses a palantír, a seeing-stone that links Saruman to Sauron and contributes to his corruption.
The tower itself survives the Ents’ attack on Isengard because its ancient stone cannot be harmed by ordinary weapons. After the fortress is destroyed and flooded, Saruman becomes trapped inside Orthanc until Treebeard releases him. Cirith Ungol and Minas Morgul are fortresses in Mordor, while Barad-dûr is Sauron’s tower; none is Saruman’s home.