Beginning in the 2nd century, London - or Londinium as it was known - was surrounded by a wall established by the Romans. This served for defence, security and status. Sections of the wall can still be seen around the city. 

This particular section near the Tower of London was part of a gate tower, and is believed to have been built by royal masons in around the 13th century. It's hard to comprehend just how old it is!