The Herodian-era tower, partially preserved in the Citadel of Jerusalem has been identified as either the Phasael Tower or the Hippicus Tower described by Josephus. It was situated close to where the Jaffa Gate is today and was built by Herod the Great at the same time he built his immediately adjacent royal palace. During the Byzantine period, the remaining tower, and by extension the Citadel as a whole, acquired its alternative name - the Tower of David - after the Byzantines, mistakenly identifying the hill as Mount Zion, presumed it to be David's palace mentioned in 2 Samuel 5:11, 11:1-27, 16:22.
Of the original tower itself, some sixteen courses of the original stone ashlars can still be seen rising from ground level, upon which were added smaller stones in a later period, which added significantly to its height. Herod named the tallest of the towers, 145 feet in height, the Phasael in memory of his brother who had committed suicide while in captivity. On the site itself, from the top of the Hippicus (or Phasael) Tower, there are good views over the excavations inside the Citadel and out to the Old City of Jerusalem, as well as into the distance south and west.
The Phasael tower was one of the three towers situated in the northwest corner of the First-wall, other two being the Hippicus and Mariamne. These towers were built in to the Hasmonean and Herodian city wall protecting the Western Hill of Jerusalem. These towers protected the main entrance to the city, as well as the palace, constituting a potential last refuge for the king. All three towers have vanished except for the base of the Hippicus (or Phasael) Tower, upon which the present "Tower of David" rests.
As evidenced by the archaeological discovery of the Broad Wall, King Hezekiah was the first to fortify this area. The city's fortifications demonstrate that by the late eighth century the city had expanded to include the hill to the west of the Temple Mount. The motivation for building the walled fortification was the expected invasion of Judea by Sennacherib. The wall might be the one referred to in Nehemiah 3:8 and Isaiah 22:9-10.
When the city was razed in 70 CE, all three towers were left standing, in order to show off the strength of the fortifications the Roman army had to overcome. With time though, only one of Herod's towers survived - either Phasael, or according to some, including archaeologist Hillel Geva who excavated the Citadel, Hippicus. The most beautiful of the three towers was the Mariamne Tower named after his favorite wife. The Hippicus Tower was named after his friend.
...Caesar gave orders that they should now demolish the entire city and temple, but should leave as many of the towers standing as were of the greatest eminence; that is, Phasael, and Hippicus, and Mariamme, and so much of the wall as enclosed the city on the west side. This wall was spared, in order to afford a camp for such as were to lie in garrison; as were the towers also spared, in order to demonstrate to posterity what kind of city it was, and how well fortified, which the Roman valour had subdued. - Josephus Flavius, Josephus, The Jewish War, 7.1.1.
After the Arab conquest of Jerusalem in 638, the new Muslim rulers refurbished the citadel. The Citadel was gradually built up under Muslim and Crusader rule and acquired the basis of its present shape in 1310, under the Mamluk sultan Malik al-Nasir.
Suleiman the Magnificent later constructed the monumental gateway in the east that allows entry in to the Jerusalem citadel complex, today. The minaret (no public access), a prominent Jerusalem landmark, was added between 1635 and 1655, and took over the title of "Tower of David" in the nineteenth century, so that the name can now refer to either the whole Citadel or the minaret alone.