Orthanc (S. ‘Forked height’; Rohirric: ‘the cunning mind’) was the tower of Isengard (previously Angrenost), sometimes referred to as the Pinnacle of Orthanc.
Built by Men of Gondor following the founding of the Realms in Exile in the Second Age, the tower stood about 500 feet above the plain of Isengard, and had been built from four pillars of unbreakable black rock. For years the impenetrable Tower was of great use to Gondor, but sometime in the late Third Age, the Dúnedain had withdrawn their forces, and the Tower remained locked as the Ring of Angrenost slowly fell into despair.
The Tower remained deserted until TA 2759, when Saruman settled there, lured by one of the palantíri which was located somewhere inside. After much searching, he found the palantír of Orthanc, and with it in his possession, he gained much strength and knowledge.
However, at the end of the War of the Ring, his Tower was destroyed by the Ents and he was expelled from the Tower, which was then passed on to the kingdom of Gondor, its rightful owner.