London has many iconic landmarks including Big Ben, Trafalgar Square with Nelson's Column, the National Gallery, Buckingham Palace, Westminster Abbey, the Tower of London, and Tower Bridge. Trafalgar Square is the central square of the city and features the National Gallery which houses European paintings. Buckingham Palace is the Queen's official residence and tourists often visit to see the Changing of the Guard ceremony. The Tower of London sits across the river from Tower Bridge and was originally built as a fortress and royal palace before becoming a prison and now a museum of arms.