The City of London is a geographically-small city within Greater London. It is the historic core of London from which, along with Westminster, the modern conurbation grew. The City's boundaries have remained constant since the Middle Ages, and hence it is now only a tiny part of the larger London metropolis. The City of London is today a major business and commercial centre, ranking just below New York City as the leading centre of global finance. It is often referred to as just the City or as the Square Mile, as it is approximately one square mile (2.6 km²) in area. Here, modern office blocks contrast with the churches and buildings of historical London, giving the area a unique and interesting vibe. The ?city? is also home to two of the great sights of London, Tower Bridge and The Tower of London, both very popular attractions.