In the early 1970s, an archaeological mission headed by Professor Nahman Avigad unearthed part of the city wall from the period of King Hezekiah (end of the 8th century BCE).
Evidently the city's population had increased even before Hezekiah ascended the throne, and Jerusalem expanded to the slopes of Mount Zion. When the Assyrian army approached, the king decided to fortify the city and wall in the newly built areas. The archaeological dig found that houses which had stood on the planned route of the wall were demolished and their stones used to reinforce the wall.
According to Isaiah (22:9-10): "And you took note of the many breaches in the City of David... and you counted the houses of Jerusalem and pulled houses down to fortify the wall." The spare description conveys extraordinarily well the atmosphere of urgency and drama that accompanied the building of the wall. It was a vast project which, together with the hewing of the water.
Sources: The Jerusalem Mosaic. Copyright 1995 Hebrew University of Jerusalem -- All Rights Reserved.