Circle of Isengard

A thick, rocky wall that enclosed a small inner vale within the larger valley of Nan Curunír. Also called the Ring of Isengard, the wall entirely enclosed a circular area a mile from edge to edge. It was here that the earliest Gondorians constructed one of the great defences of their realm: on a rock in the middle of the circle they raised the impregnable Tower of Orthanc to guard the Gap of Calenardhon and the western borders of their realm. They also made a single gateway through the rock of the Circle.

After the coming of the Rohirrim long afterwards, Isengard became an isolated Gondorian outpost far from the borders of the kingdom itself. In III 2759 Steward Beren of Gondor agreed to give control of the complex to Saruman. That Wizard would hold it until the War of the Ring, building an army within the circular walls. He was ultimately defeated, and the Ents remade the interior of the Circle of Isengard to create parkland they called the Treegarth of Orthanc.

It seems clear that the Circle was a natural formation, and the builders of Gondor merely took advantage of its ideal defensive structure. We're not told how this remarkable formation originated, but its circular shape and the fact that it contained a central mound of rock strongly suggest how it was originally formed. These factors point towards it being the crater of a meteorite or something of the same kind.

Such impacts are not unknown in the history of Middle-earth. Beleg's sword Anglachel (later called Gurthang) was forged from remarkably strong iron 'that fell from heaven as a blazing star' (Quenta Silmarillion 21, Of Túrin Turambar), a fact that echoes the remarkable strength of the Circle and its central Orthanc-rock. Indeed, since we're given no details about the origins of the sword's meteoric iron, it is not impossible that it came from the very meteorite that created the Circle of Isengard.