With its neo-Gothic towers and sky-blue suspension struts, Tower Bridge is one of London's most recognisable sights. London was a thriving port in 1894 when it was built as a much-needed crossing point in the east, equipped with a then-revolutionary steam-driven bascule (counterbalance) mechanism that could raise the roadway to make way for oncoming ships in just three minutes.
The story of building the structure is recounted in the Tower Bridge Exhibition. The bridge is still operational, although these days it's electrically powered and raises mainly for pleasure craft. Tower Bridge is in action around 1000 times a year and as often as 10 times a day in summer; check the website for lift times.