'Iron Fortress', Númenórean fortress on the west borders of Gondor, afterwards inhabited by the wizard Curunir (Saruman), see Isengard.
Angrenost was built just prior to the end of the SeA, and while it remained under the control of Gondor its chief purpose was to keep watch over the Gap of Rohan, a task it shared with Aglarond (the Hornburg) to the south, and prevent any incursions from Eriador through this easiest of entrances into Gondor. But it is told that during the Watchful Peace (ThA 2063-2460) the peoples of Calenardhon dwindled, and the Gondorian garrisons at Angrenost fell into the hands of a hereditary chieftain, whose people became less and less concerned with the affairs of their kin in Minas Tirith; and over time, as Dunlendings spread into the Westfold through the Gap, intermarried with them. The Tower of Isengard, however, remained locked; the keys were kept with the Stewards in Minas Tirith.
When the Rohirrim settled in Calenardhon, they drove many of the Dunlendings back through the Gap (and so earned their enmity). But they kept away from Angrenost--or Isengard, as they called it--fearing the "Lord of Isengard" and his secretive folk, who they believed to be practicioners of dark arts. It was soon discovered (c. 2710) that Dunlendings had assumed complete control over the Ring of Isengard, and King Déor of Rohan was alarmed to learn that he had now to deal with an enemy stronghold on his western borders. Unable to secure the assistance of Gondor, Déor did not risk taking Isengard by force. Over the next fifty years the Rohirrim maintained a heavy presence in Westfold, until the Orc, Haradrim and Dunlending invasions of 2758-59. In the process of defeating these enemies, the new king Fréalaf besieged Isengard and starved out its occupants.
It was at about this time that Saruman expressed an interest in taking up residence in the complex. Fréalaf of Rohan and Beren of Gondor assented, believing Saruman to be a true friend and ally, and he was given the keys to Orthanc. And so was established the realm of the new Lord of Isengard.