"I rode away at dawn; and I came at long last to the dwelling of Saruman. That is far south in Isengard, in the end of the Misty Mountains, not far from the Gap of Rohan. ... 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."
—Gandalf, The Fellowship of the Ring
Isengard, also known as "Angrenost" (Iron Fortress) in Sindarin, was one of the three Fortresses of Gondor and held within it one of the realm's Palantiri. However, in the latter half of the Third Age, it was entrusted to Saruman, and it then became his personal realm until his defeat during the War of the Ring.