OrthancOrthanc - The abode of Saruman in the Third Age, but not built by him.

 

"These were the chief dwellings of the Númenóreans in Gondor, but other works marvelous and strong they built in the land in the days of their power, at the Argonath, and at Aglarond, and at Erech; and in the circle of Angrenost, which Men called Isengard, they made the Pinnacle of Othanc of unbreakable stone." ~The Silmarillion (Of the Rings of Power and the Third Age)

 

But with the arrival of the Istari, Saruman took up residence in the tower.

 

"This was Othanc, the citadel of Saruman, the name of which had (by design or chance) a twofold meaning; for in the Elvish speech orthanc signifies Mount Fang, but in the language of the Mark of old the Cunning Mind.  A strong place and wonderful was Isengard, and long it had been beautiful; and there great lords had dwelt, the wardens of Gondor upon the West, and wise men that watched the stars.  But Saruman had slowly shaped it to his shifting purposes, and made it better, as he thought..." ~The Two Towers, Book III, chap. 8 (The Road to Isengard)