Only 9,000 people sleep in this area but it accommodates the activity of nearly 320,000 workers during the day. The term 'City' is usually used to refer to the financial district, which is one of the most important in the world, although it actually spans around two kilometres in diameter, situated between Westminster, Southwark and the Thames. If you go around it, near the Tower of London, you'll find some remains of the defensive wall which was built by the Roman Empire and marks out this area. Although you usually don't believe that there will be much to see in the financial district, don't hold yourself back from taking a walk around it, and most of all during the week because the bustle of executives and business people will transport you from a city full of history and restraint, to another one which is young and remarkably alive. Here is where you'll be able to see the contrast between this city's predominantly old appearance and its more modern side. The tall buildings and skyscrapers here stand out from the old constructions to the west of the city. Divert your walk a little, if you need to, to approach the church of St. Helen and you will be able to have a tremendously powerful, graphic idea of this historical distance, once you see the small medieval building guarded by the enormous and modern building St. Mary Axe 30.