The Tower Bridge was constructed at the end of the XIX century. ‘Under the Corporation of London (Tower Bridge) Act 1885, the City of London Corporation is required to raise the Bridge to provide access to and egress from the Upper Pool of London for registered vessels with a mast or superstructure of 30 feet or more. The service is provided free of charge subject to 24 hours’ notice and is available any time, day or night, 365 days per year. The Bridge is raised around 850 times each year.’ (citation from: towerbridge.org.uk)
Making short research on the bridge before posting the photo I discovered that you can actually make a sightseeing tour inside the bridge, which is not simple walking there and back. There are even tours for engineers. You can dine there renting one of three lounges for a private or a corporate event. A wedding on a tower bridge! Wow!