Isengard

The fortress of Isengard was located in a valley at the southern tip of the Misty Mountains; the Ring of Isengard was a great wall of stone that ran out from the mountain-side and then back again. The only entrance to Isengard was found delved into the southern wall of Isengard. One entering Isengard would pass through its only gate, travel down the long tunnel and pass through the iron doors to come to the plain of Isengard. In the centre of the plain, half a mile from the gate, stood the looming tower of Orthanc. For most of its history the lands of Isengard were green and a river ran from the mountains to form a lake. However after the treason of Saruman all green things that grew were removed and the river was dammed. The inner-ring was riddled with houses which cut into the walls. About the plain great shafts had been dug deep and many stone-flags paved the roads of the surface.

Layout
"A great ring-wall of stone, like towering cliffs, stood out from the shelter of the mountain-side, from which it ran and then returned again. One entrance only was there made in it, a great arch delved in the southern wall. Here through the black rock a long tunnel had been hewn, closed at either end with mighty doors of iron. They were so wrought and poised upon their huge hinges, posts of steel driven into the living stone, that when unbarred they could be moved with a light thrust of the arms, noiselessly. One who passed in and came at length out of the echoing tunnel, beheld a plain, a great circle, somewhat hollowed like a vast shallow bowl: a mile it measured from rim to rim."

-Uknown Visiter of Isengard