Dr. Eliat Mazar, the archaeologist who has been working on the site believed to be David's palace, made an announcement recently at a professional conference that she thinks she has discovered part of Nehemiah's wall. Nehemiah was the cupbearer to King Artaxerxes who received permission to return to Jerusalem and head up a building project to rebuild the walls of the city. In ancient times, a city without walls was completely defenseless from attack.
I am going to link to Todd Bolen's blog entry about this research because it's way more informed than anything I could write.
Excavator Claims Nehemiah's Wall Found in Jerusalem