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 of 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.
--The Two Towers, book III, chapter 8: The Road to Isengard
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.
--The Two Towers, book III, chapter 10: The Voice of Saruman