But Isengard is a circle of sheer rocks that enclose a valley as with a wall, and in the midst of that valley is a tower of stone called Orthanc. It was not made by Saruman, but by the Men of Númenor long ago; and it is very tall and has many secrets; yet it looks not to be a work of craft. It cannot be reached save by passing the circle of Isengard; and in that circle there is only one gate.
...
`They took me and they set me alone on the pinnacle of Orthanc, in the place where Saruman was accustomed to watch the stars. There is no descent save by a narrow stair of many thousand steps, and the valley below seems far away.

(LoTR, II-2)

Westward he looked and saw the broad pastures of Rohan; and Orthanc, the pinnacle of Isengard, like a black spike.

(LoTR, II-10)

'What is Isengard like?' said Pippin. 'I wonder what Ents can do about it anyway.' 'So do I,' said Merry. 'Isengard is a sort of ring of rocks or hills, I think, with a flat space inside and an island or pillar of rock in the middle, called Orthanc. Saruman has a tower on it. There is a gate, perhaps more than one, in the encircling wall, and I believe there is a stream running through it; it comes out of the mountains, and flows on across the Gap of Rohan.

(LoTR, III-4)

To the centre all the roads ran between their chains. There stood a tower of marvellous shape. It was fashioned by the builders of old, who smoothed the Ring of Isengard, and yet it seemed a thing not made by the craft of Men, but riven from the bones of the earth in the ancient torment of the hills. A peak and isle of rock it was. black and gleaming hard: four mighty piers of many-sided stone were welded into one, but near the summit they opened into gaping horns. their pinnacles sharp as the points of spears, keen-edged as knives. Between them was a narrow space, and there upon a floor of polished stone, written with strange signs, a man might stand five hundred feet above the plain. This was Orthanc, the citadel of Saruman, the name of which had (by design or chance) a twofold meaning; for in the Elvish speech orthanc signifies Mount Fang, but in the language of the Mark of old the Cunning Mind.

(LoTR, III-8)

They roared and boomed and trumpeted, until stones began to crack and fall at the mere noise of them. Merry and I lay on the ground and stuffed our cloaks into our ears. Round and round the rock of Orthanc the Ents went striding and storming like a howling gale, breaking pillars, hurling avalanches of boulders down the shafts, tossing up huge slabs of stone into the air like leaves. The tower was in the middle of a spinning whirlwind. I saw iron posts and blocks of masonry go rocketing up hundreds of feet, and smash against the windows of Orthanc. But Treebeard kept his head. He had not had any burns, luckily. He did not want his folk to hurt themselves in their fury, and he did not want Saruman to escape out of some hole in the confusion. Many of the Ents were hurling themselves against the Orthanc-rock; but that defeated them. It is very smooth and hard. Some wizardry is in it, perhaps, older and stronger than Saruman's.
Anyway they could not get a grip on it, or make a crack in it; and they were bruising and wounding themselves against it.

(LoTR, III-9)

They passed through the ruined tunnel and stood upon a heap of stones, gazing at the dark rock of Orthanc, and its many windows, a menace still in the desolation that lay all about it.
...
They came now to the foot of Orthanc. It was black, and the rock gleamed as if it were wet. The many faces of the stone had sharp edges as though they had been newly chiselled. A few scorings. and small flake-like splinters near the base, were all the marks that it bore of the fury of the Ents.
On the eastern side, in the angle of two piers, there was a great door, high above the ground; and over it was a shuttered window, opening upon a balcony hedged with iron bars. Up to the threshold of the door there mounted a flight of twenty-seven broad stairs, hewn by some unknown art of the same black stone. This was the only entrance to the tower; but many tall windows were cut with deep embrasures in the climbing walls: far up they peered like little eyes in the sheer faces of the horns.

(LoTR, III-10)

These were the chief dwellings of the Númenóreans in Gondor, but other works marvellous and strong they built in the land in the days of their power, at the Argonath, and at Aglarond, and at Erech; and in the circle of Angrenost, which Men called Isengard, they made the Pinnacle of Orthanc of unbreakable stone.

(Silm, OF THE RINGS OF POWER AND THE THIRD AGE)

Orthanc - 'Forked Height', the Númenórean tower in the Circle of Isengard. 361-2, 372

anca - 'jaws' in Ancalagon (for the second element in this name see alqua).

orod - 'mountain' in Orodruin, Thangorodrim; Orocarni, Oromët. Plural ered in Ered Engrin, Ered Linden, etc.

(Silm, Appendix)