Before the outbreak of bubonic plague in the early 20th century and the subsequent slum clearances, this section of the Rocks was a warren of houses connected by tiny lanes. It then spent decades covered by a car park, until 1994 when archaeologists commenced a 15-year dig, uncovering little treasures along the way. Displays bring the ruins to life.
Some of the finds from the site can be seen at the Rocks Discovery Museum.
It's worth browsing the website for more background information, too.