The so-called Tower of Fasail is on the photo. The lower part of the tower is made of large stones dates back to the 1st century BC, the upper partof the tower was constructed in the Mameluke period of Jerusalem.
The Romans, as already was noted, made the Citadel their stronghold with a garrison town; under the Byzantines there was a monastery inside of it. The Crusaders drove out the monks and the Citadel became a citadel again, with a deep ditch appearing around it. The Crusaders, by the way, were never able to take it during the assault on Jerusalem; the garrison of the Citadel got an so called honorable surrender with the preservation of personal weapons.