Jerusalem´s Old City Walls encompass an area of barely 1 km². They were built in the 16th century (1535-1538) by the Sultan Suleiman the Magnificent of the Ottoman Turks after they had been razed in 1219 by al-Mu´azzim. Those are the walls mentioned when talking about Jerusalem´s walls, but there have been many different sets of walls surrounding Jerusalem.
Before King David´s conquest of Jerusalem in the 10th century BC the city was known as Jebus and the home of the Jebusites. The Bible describes the city as heavily fortified with a strong city wall but not much is known about it.
King Solomon extended the city walls. In about 440 BC, under the Persian period, Nehemiah arrived from Babylon and rebuilt them.
In AD 41-44 Agrippa, king of Judea, built a new city wall known as the "Third Wall".
In 135, after Emperor Hadrian´s total destruction of Jerusalem and renaming it to Aelia Capitolina he rebuilt the walls.
The current walls of the Old City do not include the original area conquered by King David. This area, which is now called the City of David, is located to south east of the current Old City Outside The Dung Gate.
Taken from Wikipedia.org