City of London London borough, a status reserved for the other 32 districts (including London's only other city, the City of Westminster). It is also a separate county of England, being an enclave surrounded by Greater London. It is the smallest county in the United Kingdom. The City of London is widely referred to simply as the City (differentiated from the phrase "the city of London" by capitalising "City") and is also colloquially known as the Square Mile, as it is in area. Both of these terms are also often used as metonyms for the United Kingdom's trading and financial services industries, which
City of London Bridge, Tower Bridge and the Millennium Bridge. The City's flag flies over Tower Bridge, although neither footing is in the City. 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