In J.R.R. Tolkien's world of The Lord of the Rings, Isengard was a fortress built by the men of Gondor in the days of its power in Nan Curunir. Isengard consisted of a natural large ring wall of stone, which surrounded a broad plain with great trees. In the center of this was the tower of Orthanc. Isengard had only one gate in its wall, which faced the South.
When Calenardhon was given to the Rohirrim in TA 2510, Gondor kept Isengard, but left it deserted. The Dunledings took it in 2700, but in 2759 Frealaf drove them out. After its recapture in that same year Saruman gained permission from Beren of Gondor to live in Isengard, but in 2963 he took it as his own and began to fortify it. He replaced the grass and trees from the plain and replaced them with stone and machines. His minions were Orcs, men and wolves who did his bidding and the bidding of Sauron from Isengard. He bred Uruk-Hai as well to fight the Rohirrim and other forces of good.
The Ents went to Isengard with Merry and Pippin, destroyed the wall, killed the evil creatures that dwelt there and flooded the plain. Saruman and Grima Wormtongue remained in Orthanc until Gandalf took Saruman's powers away and broke his staff. After The War of the Ring the Ents planted the Watchwood there and renamed Isengard the Tree-garth of Orthanc.
Also called Angrenost, or the Ring or Isengard.