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Constructed between 1886-1894, Tower Bridge is one of London’s most recognized sights. It is the first of London’s Thames 1_______ and the only one which can allow the passage of shipping underneath. The bridge was 2_______ to be a two-level span: the lower level, a draw bridge which elevates to permit tall river traffic, and the upper level, twin walkways for pedestrians. The observation 3_______ in the north tower permits superb views of the City of London and the surrounding Docklands. The Tower Bridge Museum in the south tower contains the original steam-driven 4_______ that was used to raise the bridge.