As a result of the unjustified beating of Paul by the Jews in the temple precinct, Paul was rescued by the commander of the Roman cohort. Orders were given for Paul to be bought into the barracks. The KJV and the NKJV use the word castle. The CEB use of military headquarters probably conveys the correct idea. (Acts 21:27-40). The parembole served as headquarters for the Roman troops in Jerusalem (BDAG).
The building under consideration in Acts 21 is the Fortress of Antonia. The massive structure was built by Herod the Great and named in honor of Mark Anthony. Murphy-O’Connor says the fortress “both protected and controlled the Temple” (The Holy Land, 34). The temple was surrounded by the Kidron Valley on the east, the Tyropean Valley on the west, and a steep decline on the south. The area needing special protection was the northwest corner.
Josephus describes the “tower of Antonia” in the Wars of the Jews.
238 Now, as to the tower of Antonia, it was located at the corner of two cloisters of the court of the temple; of that on the west, and that on the north; it was erected upon a rock of fifty cubits in height, and was on a great precipice; it was the work of King Herod, wherein he demonstrated his natural magnanimity. 239 In the first place, the rock itself was covered over with smooth pieces of stone, from its foundation, both for ornament, and that anyone who would either try to get up or to go down it might not be able to hold his feet upon it. 240 Next to this, and before you come to the edifice of the tower itself, there was a wall three cubits high; but within that wall all the space of the tower of Antonia itself was built upon, to the height of forty cubits. (JW 5:238-240)
The photo above shows the Tower of Antonia built on the natural rock foundation at the northwest corner of the temple precinct. A portion of the Pool of Bethesda (John 5) is visible on the right.
Paul’s speech recorded in Acts 22 took place on the stairs leading up from the temple platform into the fortress. When the crowd drowned out Paul, the commander ordered that he be brought into the fortress (22:24). After Paul’s nephew informed the Roman officers about a planned conspiracy to kill Paul, he was sent to Caesarea where he would remain for two years (23:23; 24:27).
The photo below was made from inside the temple precinct. It shows the natural rock scarp in the northwest corner on which the Fortress of Antonia was built. See Leen Ritmeyer’s post about this, and his drawing showing the location of the steps mentioned in Acts 21. Ritmeyer sells digital images of his drawings.