The City or the Square Mile was once the old city contained within mediaeval walls. Now it is the nation's financial centre and embodies London's mix of tradition and change. This tiny area, only twenty minutes walk from one side to the other, generates 3% of the UK's income. The City by night is among the most stunning sights in London, a ghost town of towering skyscrapers and stunning churches, including St. Paul's Cathedral, the spiritual heart of Great Britain. Around the edges of the city are some of the best arts and entertainment venues in London, including Fabric nightclub, Smithfields Market, and the wonderful Barbican Arts Centre, where much of the capital's best classical and contemporary music, theatre, and art can be found.