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. In the fantasy world, the name of the fortress is described as a translation of Angrenost, a word in Tolkien's elvish language, Sindarin, a compound of two Old English words: īsen and ġeard, meaning "enclosure of iron".

In The Lord of the Rings, Orthanc, the tower at the centre of Isengard, is the home of the Wizard Saruman. He had been ensnared by the Dark Lord Sauron through the tower's palantír, a far-seeing crystal ball able to communicate with others like it. Saruman had bred Orcs in Isengard, in imitation of Sauron's forces, to be ready for war with Rohan. The Orcs cut down many trees in the forest of the Ents, who retaliate by destroying Isengard while the army of Orcs is away attacking Rohan at Helm's Deep. However, the Ents are unable to harm the tower of Orthanc. Saruman, isolated in the tower, is visited by members of the Fellowship of the Ring; his staff is broken by the Wizard Gandalf.

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. Others have compared it to Vichy France, and its proposed governor on behalf of Mordor, the Mouth of Sauron, to a traitorous Quisling.

The Númenóreans in exile built Isengard in the Second Age as a walled circular enclosure, with the tower of Orthanc at its centre. 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.

The river Isen or Angren began on Methedras, the southernmost peak of the Misty Mountains. Methedras stood behind Isengard, forming its northern wall. The rest of its perimeter consisted of a large wall, the Ring of Isengard, breached only by the inflow of the river at the north-east through a portcullis, and the gate of Isengard at the south, at both shores of the river. For most of its history, Isengard was a green and pleasant place, with many fruiting trees.

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. No known weapon could harm it. Orthanc rose to more than 500 feet (152 metres) above the plain of Isengard, and ended in four sharp peaks. Its only entrance was at the top of a high stair, and above that was a small window and balcony. It housed one of the palantírs of the South Kingdom, and was guarded by a warden.

In the Third Age the land around Isengard (Calenardhon) became depopulated, and the last warden of Orthanc was recalled to Minas Tirith. Isengard remained guarded by a small company, led by a hereditary captain. Contact with Minas Tirith gradually decreased and eventually ceased altogether. When Cirion, Steward of Gondor, gave Calenardhon to the Éothéod, becoming the land of Rohan, Isengard was the sole fortress retained by Gondor north of the Ered Nimrais. The small guard intermarried much with the Dunlendings, until the fortress became Dunlending in all but name. 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. Using Isengard as their base, the Dunlendings continually raided Rohan until during the rule of Helm Hammerhand, the Dunlending lord Freca and his son Wulf nearly managed to destroy the Rohirrim. The Rohirrim fought off the invaders and blockaded Isengard, eventually taking it.

Gondor did not wish to relinquish its claim to the tower, but lacked the strength to garrison it. A solution presented itself to the Steward of Gondor, Beren, as the Wizard Saruman suddenly reappeared from the East, offering to guard Isengard. Beren gladly gave him the keys to Orthanc. At first he resided there as Warden of the Tower on behalf of Gondor. The valley became known as Nan Curunír, the "Wizard's Vale". On Sauron's return to Mordor, Saruman asserted himself as Lord of 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.

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. A carved and painted White Hand of stone is set on a black pillar outside the gates of Isengard.

Treebeard, leader of the Ents, seeing that the Orcs would destroy his forest of Fangorn, leads an army of Ents and Huorns to Isengard, destroys it, and floods it, leaving Saruman isolated in the tower of Orthanc. 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. Gandalf speaks with Saruman and breaks his staff. Grima Wormtongue throws the Orthanc palantír, a stone of seeing, at the party; both Pippin and Aragorn later use it, seeing and deceiving Sauron as to the Fellowship's intentions.

Saruman is locked in Orthanc and guarded by Treebeard, who later sets him free. Saruman hands the tower's keys over to Treebeard, and takes Gríma with him. Saruman exploits Treebeard's unwillingness to see any living thing caged, most likely using his power with words.

During the Fourth Age, when Aragorn has been crowned as King Elessar ("Elfstone"), he visits Orthanc, finding there heirlooms of Isildur, among them the Elendilmir, the Star of Arnor, and the small gold case on a chain that Isildur had used to carry the One Ring, evidence that Saruman had found and apparently destroyed Isildur's remains. Isengard is restored, and the entire valley granted to the Ents. The Ents name the new forest the Treegarth of Orthanc. Orthanc becomes again a tower of the Reunited Kingdom of Gondor and Arnor.

"Isengard" is from Old English īsen, "iron" and geard, "court, enclosure". The names, supposedly given by the Rohirrim, for Orthanc, the cunningly-built tower of Isengard, and for the Ents, the tree-giants of Fangorn forest who eventually destroy Isengard, are similarly in reality from Old English. Both are found in the poem The Ruin, which describes the ancient Roman ruins as orþanc, "skilful work", and enta geweorc, "the work of giants" and in Maxims II. Clark Hall gives the meanings of the noun orþanc as "intelligence, understanding, mind; cleverness, skill; skilful work, mechanical art", and as an adjective "ingenious, skilful". The Tolkien scholar Tom Shippey suggests that Tolkien may have chosen to read the phrase also as "Orthanc, the Ent's fortress".

The name of the tower of Orthanc is unique in that it is explicitly stated to be a bilingual pun in The Two Towers: Tolkien gives the two meanings as "Mount Fang" in Elvish (Sindarin), and "Cunning Mind" in the "language of the Mark of Old", Rohirric. However, "Orthanc" genuinely means "Cunning Mind" in the language Tolkien had used to represent Rohirric, Old English: he had pretended that he had translated Rohirric into Old English, and the related Westron into modern English. The unlikely coincidence of homonyms and synonyms makes Tolkien's claim about Rohirric look like a mistake.

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  In The Two Towers, Tolkien said Orthanc had meanings in Sindarin and Rohirric; but it is also a synonym and homonym in Old English, making Tolkien's claim look like a mistake.

Tolkien made detailed sketches of Isengard and Orthanc, published in J. R. R. Tolkien: Artist and Illustrator, as he developed his conception of them.

The scholar of English literature Charles A. Huttar describes Isengard as an "industrial hell". He quotes Tolkien's description of Isengard, supplying his own emphasis on Tolkien's words: "tunneled .. circle .. dark .. deep .. graveyard of unquiet dead .. the ground trembled .. treasuries .. furnaces .. iron wheels .. endlessly .. lit from beneath .. venomous." Huttar comments: "The imagery is familiar, its connotations plain. This is yet another hell [after Moria and Mordor]." 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".

Shippey, discussing Saruman's character, notes several facts about him: Treebeard's comment that "He has a mind of metal and wheels"; that Isengard means "Irontown"; that the Ents are attacked in Isengard with "a kind of napalm [or] perhaps ... [given] Tolkien's own experience, a Flammenwerfer". Shippey concludes that Saruman had been led into "wanton pollution ... by something corrupting in the love of machines", which he connects to "Tolkien's own childhood image of industrial ugliness ... Sarehole Mill, with its literally bone-grinding owner".

David D. Oberhelman, writing in the J.R.R. Tolkien Encyclopedia, states, following Anne C. Petty, that there are multiple "industrial 'hells' in Tolkien's work, such as Saruman's blighted, machine-ridden Isengard". He notes that its prototype was the fallen Vala Morgoth's subterranean fortress, Angband, whose name meant "Iron Prison" or "Hell of Iron".

Isengard is the promised reward for the nameless "Mouth of Sauron", as soon as Gondor and its allies had surrendered. In his words in front of the Black Gate:

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. But they shall help to rebuild Isengard which they have wantonly destroyed, and that shall be Sauron's, and there his lieutenant shall dwell: not Saruman, but one more worthy of trust.

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."

During the War of the Ring, Isengard was controlled by Saruman until the fortress's destruction, but Saruman had become "more like Sauron than he realizes", like him believing in "supremacy through absolute power", and unintentionally a pupil of Sauron, having against Elrond's advice "stud[ied] too deeply the arts of the enemy". The Tolkien scholars Wayne Hammond and Christina Scull note that the palantír in Orthanc had formed what Gandalf called "some link between Isengard and Mordor, which I have not yet fathomed": the link was that Sauron had used the stone to take control of Saruman, and through him his forces of Orcs. 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." The Tolkien scholar Brian Rosebury writes that Tolkien was making the point that whereas good government in free societies like those of Gondor, the Dwarves, the Elves, the Drúedain, and the Shire leads to diversity, "evil tends to homogeneity".

In Peter Jackson's films of The Lord of the Rings, Isengard and Orthanc were based on Alan Lee's illustrations and modelled under the direction of Richard Taylor; Lee worked as the project's conceptual artist in New Zealand throughout the making of the film trilogy. 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. The model of the walled circular area of Isengard was more than 65 feet (20 m) wide. In post-production, the long shots of the Orthanc model were combined, using chroma keying, with panoramic views of the Mount Earnslaw / Pikirakatahi region and Mount Aspiring National Park near Queenstown and Glenorchy, New Zealand.

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