Machaerus (Greek: "The Sword") was a Hasmonean stronghold in Perea built by Alexander Jannai. As the base of Aristobulus' II resistance, it was destroyed by Pompey [64 CE] but later rebuilt by Herod on a grander scale, complete with a lavish palace and enough supplies to withstand a five-year siege. It was located five miles west of the mineral springs of Callirohe. According to Josephus it was the place where Antipas imprisoned and executed Johanan the Baptizer [Antiquities 18.118-119].
Source: Into His Own