In J. R. R. Tolkien's fantasy writings, Isengard (/ˈaɪzənɡɑːrd/) is a large fortress in Nan Curunír, the Wizard's Vale, in the western part of Middle-earth.

Isengard has been described by Tolkien scholars as an industrial hell, and as an illustration of the homogeneity of evil, in contrast to the evident diversity of the free societies of Middle-earth, including those of the Elves, Dwarves, and Gondor.

It lay just outside the north-western corner of Rohan, guarding the Fords of Isen from enemy incursions into Calenardhon together with the fortress of Aglarond to its south.

[T 2] Orthanc was built towards the end of the Second Age by men of Gondor from four many-sided columns of rock joined by an unknown process and then hardened.

The tower of Orthanc however remained locked and inaccessible to the Dunlendings, as the Steward of Gondor alone held the keys in Minas Tirith.

The line of hereditary Captains died out, and during the rule of Rohan's King Déor, Isengard became openly hostile to the Rohirrim.

A solution presented itself to the Steward of Gondor, Beren, as the Wizard Saruman suddenly reappeared from the East, offering to guard Isengard.

During the War of the Ring, Saruman prepares for war against Rohan, defiling the valley of Isengard with deep pits where he breeds large numbers of powerful warrior Orcs, Uruk-hai, smithing weapons in underground workshops full of machinery, and felling the valley's trees.

[T 2] The Orcs of Isengard bear upon their shields the symbol of a White Hand on a black field, and on their helmets an S-rune () to signify Saruman.

[T 3] The hobbits Merry Brandybuck and Pippin Took, as the new "doorwardens", receive Théoden King of Rohan, Aragorn and the wizard Gandalf at the wrecked gates.

Grima Wormtongue throws the Orthanc palantír, a stone of seeing, at the party;[T 4] both Pippin and Aragorn later use it, seeing and deceiving Sauron as to the Fellowship's intentions.

[12] All the same, he writes, the tower of Orthanc cannot but be admired, with its "marvellous shape" and wonderful, ancient strength; he supposes that for Tolkien, technology could neither be "wholeheartedly embraced nor utterly rejected".

[14] Shippey concludes that Saruman had been led into "wanton pollution ... by something corrupting in the love of machines",[14] which he connects to "Tolkien's own childhood image of industrial ugliness ... Sarehole Mill, with its literally bone-grinding owner".

In his words in front of the Black Gate:[T 10] West of the Anduin as far as the Misty Mountains and the Gap of Rohan shall be tributary to Mordor, and men there shall bear no weapons, but shall have leave to govern their own affairs.

[T 10] Shippey compares Sauron's offer to the Vichy treaty imposed on France after its surrender in 1940: "sovereignty over the disputed territory of Ithilien [East of the Anduin], the Alsace-Lorraine of Middle-earth, is to be transferred", and in the lands to the West "a demilitarized zone, with what one can only call Vichy status, which will pay war-reparations, and be governed [from Isengard] by what one can again only call a Quisling".

[19] In The Two Towers, Tolkien himself described Saruman's Isengard as "only a little copy, a child's model or a slave's flattery ... [of Sauron's] vast fortress, armoury, prison, furnace of great power, Barad-dûr".

[21] The very large miniature or "bigature" of Orthanc was cast and then carved from micro-crystalline wax by Wētā Workshop to resemble obsidian, black volcanic glass; it was made at 1/35 scale, standing some 15 feet (4.6 m) high.