In the upper valley of the Isen, the river flowed southward through the last southern heights and hills of the Misty Mountains before running out into the Gap of Rohan. In that valley, in the early years of their settlement in Middle-earth, the Dúnedain had built an impenetrable fortress ringed by rock: Angrenost, or Isengard as it was known in the language of the Rohirrim who settled there long afterwards.

More than two centuries before the War of the Ring, the Wizard Saruman came to Steward Beren in Minas Tirith, and proposed that he should be granted Isengard as a dwelling-place. Seeing advantage in having such a powerful ally guarding the Gap of Rohan, Beren agreed, and granted Saruman the Key of Orthanc. From that time on, the valley where Isengard stood became known as Nan Curunír, which literally translates as the 'Valley of Saruman'. Curunír was the Elvish equivalent of Saruman's name, but when rendered into the Common Speech, Nan Curunír is known simply as 'the Wizard's Vale'.