Isengard
Like the fortifications of Helm's Deep, the works at Isengard were made by the Dúnedain in the early days of Gondor; but unlike Helm's Deep, Isengard was not given to the Éothéod when Rohan was ceded. It was kept by Gondor, but at some point became deserted. After T.A. 2758, when Rohan was overrun and Isengard seized by Dunlendings, Saruman was given the keys to Orthanc, and the great valley surrounding Isengard was named Nan Curunír, the Valley of Saruman — the Wizard's Vale.
The Ring and Tower
Isengard lay in the western part of Nan Curunír, sixteen miles from the mouth of the valley and a mile west of the Isen. The two most notable features of Isengard were the outer Ring and the tower of Orthanc. The Ring measured one mile from rim to rim and "stood out from the shelter of the mountain-side, from which it ran and returned again." The plain within was somewhat hollowed, forming a shallow basin, in the center of which stood the tower.
Drawings by Tolkien indicated that Orthanc rose high above the rimwall. As Orthanc was over five hundred feet high, the rimwall might have been only
one hundred feet high, or perhaps less: The Ents leveled it without much difficulty. Orthanc was evidently of a much more resistant rock than the rimwall. Although the tower was fashioned by the builders of Númenor, they merely altered it; for it appeared "not made by craft of Men but riven ... in the ancient torment of the hills." It most closely resembled a volcanic plug or neck such as Shiprock, in New Mexico. If the less resistant outer rock of the cone were partially removed by erosion or quarrying, the remnant might have formed the Ring of Isengard; while the dense black basalts from the central vent could have been formed into the "four mighty piers" that the Númenóreans welded into the central tower.
In Tolkien's earliest drawings Orthanc was clearly manmade — a multi-tiered structure atop an island:
The latest sketch showed the eventual conception, in which "the [island] 'rock' of Orthanc becomes itself the 'tower.'":
However, a brief note indicated the true final vision was never drawn: a combination of the earliest and latest views, explaining Orthanc's description as "a peak and isle of rock."
The Fortification
Until T.A. 2953, twelve years after the Battle of the Five Armies, Isengard had been green and pleasant, with many groves, shaded avenues, and a pool fed by waters from the mountains; but when Saruman fortified Isengard (in rivalry to the newly rebuilt Baraddûr), the groves were cut and the pool drained. Although an early design of the Ring of Isengard had a small northern gate, this was abandoned for a single entrance: an arched tunnel bored through the rockwall in the south and closed on each end with iron gates. Within the tunnel on the left (going in) was a stair that led to the guardroom where Merry and Pippin served their friends lunch. The room seemed fairly large: It had more than one window looking into the
tunnel, held a long table, had a hearth, and (in the far wall) opened into two separate storerooms where the Hobbits had found the provender. In the far corner of one of the storerooms was a stair that wound its way to a narrow opening above the tunnel.
Inside the basin a Tolkien illustration showed eight stone paths (some lined with pillars) that radiated from Orthanc to all parts of the Ring. In the Ring were delved all the living quarters of Saruman's many servants, including stables for wolves. Thus, the basin was surrounded by thousands of windows peering over the plain. Between the radiating roads the land was dotted with numerous stone domes, which sheltered shafts and vents leading from the vast underground works: "treasuries, store-houses, armouries, smithies, and great furnaces."
In the center of the plain stood Orthanc, welded by some unknown craft into a gleaming many-sided spike of black rock so strong that even the Ents could not harm it. Its only entrance was an east-facing door reached by a high flight of twenty-seven steps. Within the tower were many windows, peering through deep embrasures. Most were shown by Tolkien above the level of the door. One large shuttered archway directly above the door opened onto the balcony from which Saruman spoke. Even higher was a window through which Wormtongue cast down the palantír. At the pinnacle of the tower the four rock piers had been honed into individual horns that surrounded the high platform on which Gandalf had been held prisoner. These sharp spikes gave Orthanc its name, the "Forked Height"; and the symmetry of the tower, encircling courtyard, and radiating roads gave fortified Isengard the appearance of one of Tolkien's heraldic devices.
After Saruman was defeated, the Ents destroyed the Ring of Isengard, flooded the basin around the feet of the tower, and planted new orchards. Once more it became green and pleasant: the Treegarth of Orthanc.
Map of Angrenost (Isengard) by Karen Wynn Fonstad: