For centuries London Bridge was the only crossing over the Thames in London. As the city grew, more bridges were built upstream, but not downstream since that area was busy with shipping. By the 19th century, London's East End was so densely populated that a bridge was urgently needed and Tower Bridge was built. It was designed with two hydraulically operated bascules that lift to allow ships to pass. The two 43m supporting towers have high-level walkways for pedestrians to cross when the bascules were raised. It took eight years to complete the construction.