Isengard, also called Angrenost (‘Iron Fortress’) in Elvish, was located at the southern end of the Misty Mountains near the start of the River Isen.

Elendil and his sons built the circular stronghold when they came to Middle-Earth from Númenor. The Ring of Isengard referred to the circular stonewall that forms the outer defense.

Within the walls of Isengard, the Númenóreans also built Orthanc ('Forked Height’ or ‘Mount Fang’), an indestructible stone tower. Isengard was granted to Saruman, who lived within the stone tower, until the Ents destroyed Isengard during the War of the Ring.
Encyclopedia entry originally written by Ringhilwen