Antonia

a fortress in Jerusalem, at the north-west corner of the temple area. It is called "the castle" (Acts 21:34; 21:37). From the stairs of this castle Paul delivered his famous speech to the multitude in the area below (Acts 22:1-21). It was originally a place in which were kept the vestments of the high priest. Herod fortified it, and called it Antonia in honour of his friend Mark Antony. It was of great size, and commanded the temple. It was built on a plateau of rock, separated on the north from the hill Bezetha by a ditch about 30 feet deep and 165 feet wide.

EBD - Easton's Bible Dictionary