Isengard was built by the Numenoreans in the Second Age around the tower of Orthanc. It was built to protect Southern Gondor, in particular Calenarhon, and was once a lush and beautiful place. The number of soldiers stationed dwindled and over time they intermarried with Dunlendings and began to attack the Rohirrim (Calenarhon had passed to Rohan). The Rohirrim were barely able to defeat them and Rohan glady accepted Saruman's offer to guard it. Saruman used his fortress to breed orcs, make weapons, and pillage Rohan. He cut down the trees and (obviously) angered the Ents. The Ents succeeded in taking Isengard, but not Orthanc which was left to Aragorn, Gandalf, Theoden, Merry, and Pippin. Treebeard was left to guard Saruman and Grima Wormtongue, although they would escape after the defeat of Sauron. Treebeard replanted many trees and the forest was named Treegarth of Orthanc. Orthanc fell under the rule of Aragorn in his new united kingdom.