Antonio's Fortress, built by Herod the Great. It is located on the Northern wall of the Temple Mount and it housed the Roman Garrison stationed at Jerusalem. The South wall of the Fortress would allow the guard to look right over into the Temple mount area. This is where Paul would have been taken when rescued from the mob by the Roman guard.
The slopes of Masada were too steep for the Roman army to scale with their siege equipment so they had slaves work for months to construct this rocky ramp up the side of the mountain fortress.