Greater London is often referred to as "the City" or "the Square Mile" and it forms the main financial district. Archaically, the urbanised area of London was known as the Metropolis. In common usage, the terms "London" and "Greater London" are usually used interchangeably. Greater London is officially divided for some purposes, with varying definitions, into Inner London and Outer London. For some strategic planning purposes it is divided into five sub-regions. Greater London is under the strategic local governance of the Greater London Authority (GLA). It consists of an elected assembly, the London Assembly, and an executive head, the Mayor of
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