The City of London is the historic heart of London. It refers to the area within the original old walled city built by the Romans around AD50. This, the oldest part of London, stretches from Temple and Fleet Street in the west to Aldgate and the Tower of London in the east, from Smithfield and the Barbican in the north to the River Thames in the south. This small area of just over one square mile is the smallest ceremonial county and a thriving business centre. The City embodies a mix of tradition and change; it is the most historic part of London, yet also the most modern; a kingdom of its own with many customs, traditions and pomp and ceremony preserved for centuries mixing easily with modern life. It is a business and financial centre with beautiful churches and tranquil green spaces where Roman ruins, medieval timber buildings, Tudor and Georgian houses sit next to iconic towering skyscrapers.