Easton's Bible Dictionary (1897)
The Black Fortress, was built by Herod the Great in the gorge of Callirhoe, one of the wadies 9 miles east of the Dead Sea, as a frontier rampart against Arab marauders. John the Baptist was probably cast into the prison connected with this castle by Herod Antipas, whom he had reproved for his adulterous marriage with Herodias. Here Herod “made a supper” on his birthday. He was at this time marching against Aretas, king of Perea, to whose daughter he had been married.
During the revelry of the banquet held in the border fortress, to please Salome, who danced before him, he sent an executioner, who beheaded John, and “brought his head in a charger, and gave it to the damsel” (Mark 6:14-29). This castle stood “starkly bold and clear” 3,860 feet above the Dead Sea, and 2,546 above the Mediterranean. Its ruins, now called M’khaur, are still visible on the northern end of Jebel Attarus.
Hastings' Dictionary of the Bible (1898)
MACHAE'RUS is not mentioned in the Bible, but is supposed to be the castle in which John the Baptist was imprisoned and beheaded. It is nine miles east of the Dead Sea. Mark 6:21-29.