The size of the City was constrained by a defensive perimeter wall, known as London Wall, which was built by the Romans in the late 2nd century to protect their strategic port city. However the boundaries of the City of London no longer coincide with the old city wall, as the City expanded its jurisdiction slightly over time. During the medieval era, the City's jurisdiction expanded westwards, crossing the historic western border of the original settlement—the River Fleet—along Fleet Street to Temple Bar. The City also took in the other "City bars" which were situated just beyond the old walled area, such as at Holborn, Aldersgate, West Smithfield, Bishopsgate and Aldgate. These were the important entrances to the City and their control was vital in maintaining the City's special privileges over certain trades.