Isengard

There stood a tower of marvellous shape. It was fashioned by the builders of old, who smoothed the Ring of Isengard, and yet it seemed a thing not made by the craft of Men, but riven from the bones of the earth in the ancient torment of the hills. A peak and isle of rock it was, black and gleaming hard: four mighty piers of many-sided stone were welded into one, but near the summit they opened into gaping horns, their pinnacles sharp as the points of spears, keen-edged as knives. Between them was a narrow space, and there upon a floor of polished stone, written with strange signs.
- The Two Towers (Book 3), Chapter 8

Lying in a vale at the southern end of the Misty Mountains, the fortress of Isengard (or Angrenost) consists of a circular wall which surrounds the great tower of Orthanc. Standing at over 500 feet tall, it was constructed by the Dúnedain in the late Second Age and has housed the Orthanc-stone, one of the nine palantíri, ever since.

After the tower of Orthanc was locked and abandoned by the Gondorians following the Great Plague of T.A. 1636, the area surrounding Isengard became home to Dunlendings hostile to the nearby Rohirrim. The Rohirrim were only able to break their strength after the Long Winter in T.A. 2759, over a millenium later.

In the same year, the wizard Saruman returned from his travels in the east of Middle-earth and took up residence in the now deserted fortress. The vale of Isengard became hereafter known as "Nan Curunír", Wizard's Vale. However, the White Wizard has long since been scheming and preparing to challenge not just the realms of Men but Sauron himself.