Easton's Bible Dictionary (1897)
A fortress in Jerusalem, at the north-west corner of the temple area. It is called “the castle” (Acts 21:34, 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.
Smith's Bible Dictionary (1863)
(from Marc Antony) (a square stone fortress or castle adjoining the northwest corner of the temple area at Jerusalem. There was a tower at each corner. It was rebuilt by Herod the Great, and named by him from Marc Antony.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
ANTO'NIA, a castle or fortress built by Herod, north-west of the temple in Jerusalem, and named by him after his friend Antony. It may be the "castle" referred to in Acts 21:34.