Isengard was built in the Second Age around the tower of Orthanc by the Númenóreans in exile, in the days of their power, in Nan Curunír (The Wizard's Vale) in the Misty Mountains. Its location was at the northwestern corner of the Kingdom of Gondor, guarding the Fords of Isen from enemy incursions into Calenardhon and, together with the fortress of Aglarond to its south, protecting the Gap of Calenardhon.

Isengard was comprised of a black, circular, stone-wall surrounding a broad plain, in the center of which was the Tower of Orthanc. Isengard had only one gate, which faced south.

The River Angren (or Isen) began at Methedras behind Isengard, which also formed its northern wall. The other three sides were guarded by a large wall, known as the Ring of Isengard, which was only breached by the inflow of the River Angren at the northeast through a portcullis, and the gate of Isengard at the south, at both shores of the river. The Gate was said to be able to open without a sound.

Isengard was, at first, a green and beautiful tower complex, with many large trees and grass fields, fed by the river until it was dammed by Saruman. After the Wizard's defeat, however, Treebeard and the rest of the Ents made it a vivid green area again, after destroying the walls and dam, renaming it the Treegarth of Orthanc.

In the centuries following the Great Plague the land of Calenardhon became depopulated and the last Warden of Orthanc was recalled to Minas Tirith. Isengard remained garrisoned by a small company, led by a hereditary captain, much as happened at Aglarond at the time. At first messengers were still sent to the fortress by the Stewards, although these dwindled and then eventually ceased entirely.

In TA 2510, after Calenardhon was given to the Éothéod by Cirion, Steward of Gondor, and became Rohan, Isengard remained part of Gondor, and the company of Aglarond removed to the northern fortress, although the rest of Gondor had virtually forgotten it. The small guard intermarried much with the Dunlendings, until the place became a Dunlending fortress in all but name. Orthanc, however, remained closed, as the Steward of Gondor held the keys. Isengard soon became all but deserted.

During the rule of Rohan's King Déor, in TA 2710, the line of hereditary captains commanding the fortress died out and Isengard became openly hostile to the Rohirrim. Using Isengard as their base, the Dunlendings continued to raid Rohan during the reign of Déor's son Gram, until during the rule of Gram's son Helm Hammerhand a Dunlending lord, Freca and his son Wulf nearly managed to destroy the Rohirrim. The Rohirrim eventually won, taking Isengard by starving out the Dunlendings in TA 2759 and guarding it for Gondor.

At this time Saruman the White suddenly reappeared from the East, and he offered to guard Isengard. The Steward of Gondor Beren gladly gave him the keys to Orthanc, and Saruman settled there. After him the valley became known as Nan Curunír, or "Wizard's Vale". At first Saruman was merely Warden of the Tower on behalf of Gondor, but as his pride grew and fall began, he took the fortress for his own in TA 2953.

During the War of the Ring, Isengard was Saruman's base of operations against the Rohirrim, and he defiled the valley, cutting down its trees and grass and replacing them with stone and machinery. He dammed and diverted the Isen, possibly for use as hydropower. Isengard's valley was honeycombed with deep pits, used for breeding Orcs and forging weapons. Eventually, an army of Ents and Huorns led by Treebeard of Fangorn, attacked Isengard, wrecking all of Saruman's industries, though unable to force entry into or damage Orthanc.

The Hobbits Meriadoc Brandybuck and Peregrin Took, as the new "doorwardens", received Théoden King of Rohan, Aragorn, and Gandalf at the gates. Saruman was confronted but refused to beg for pardon, and he was kept under Treebeard's guard together with his servant Gríma Wormtongue. Saruman abandoned Isengard after Sauron's defeat, playing off Treebeard's hatred of caging any living thing in order to manipulate him into letting him leave. It was planned by Sauron that Isengard would be given to his loyal ambassador, the Mouth of Sauron, after Saruman's demise.

Early in the Fourth Age, Isengard was restored and Treebeard replanted many trees. Orthanc again became a tower of the Reunited Kingdom of King Aragorn II Elessar.

Under the Ent dominion, the ring-wall was torn down and the Ents planted the Watchwood there, and renamed the new forest the "Treegarth of Orthanc". Saruman's pits and caverns were filled, and the land was healed. The Valley was now filled with trees and a small lake was constructed near Orthanc. The valley of Nan Curunír was given to the Ents by Aragorn, to do with as they wished, granted that they keep watch upon Orthanc and prevent anyone without permission to enter it

The Tower of Isengard

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