Underneath so-called old Jerusalem, lies ancient Jerusalem. Archaeologists have excavated certain areas to expose the even older portions of the city.
If I have this correct, what's currently regarded as old Jerusalem is about 1,500 or so years old. If you think the columns seen here look like something Roman, you'd be correct, as they once occupied what is now Jerusalem.
These Roman columns are presented in a way which mimics the original layout. We're told this area was used for the open air markets; vendor stalls would have occupied the space between wall and columns. Of course, these columns are now under the 'current' old Jerusalem, but would have been the top architectural tier back in the day.
Just around the corner from these columns is a current archaeological dig exposing other Roman ruins which have been dated back to 8 centuries B.C.