Adventures in Arda
Angrenost was built in the Second Age by the Númenóreans. Together with Aglarond the two fortresses prevented any undesired passage into Gondor from Eriador.
Angrenost was built around the tower of Orthanc which is a tower of black stone. The tower has a single entry with one stairway leading to it. Higher up there is a small window and balcony. The top of the tower terminates in 4 points. The tower was locked and the keys were removed to Minas Tirith.
Around the tower the Ring of Isengard was built. This wall closes around Orthanc with the peak Methedras on the north side. The wall has only two breaches where the Isen river passes from the peak and exits towards the South.
During the Watchful Peace, the numbers in Angrenost began to dwindle and the Gondorian guards began to intermarry with the Dunlendings. Much of the guard were recalled to Minas Tirith and the fortress was left to the hands of a few hereditary captains that futher mixed with Dunlending blood. When Rohan was created, Angrenost remained the only Gondorian fortress in the North. Eventually the line of captains died out and the fortress became ruled by the Dunlendings although they could not enter the tower itself. By T.A. 2710 the Dunlendings had become openly hostile towards the Rohirrim again.