London has two main airports, Heathrow and Gatwick, that receive many tourists. Londoners use the Underground, buses, and trains to commute to work in the city. Some of London's most famous sights that people visit include the Tower of London, a former castle and prison that is now a museum housing the Crown Jewels; Tower Bridge, the iconic bridge across the River Thames; Buckingham Palace, where Queen Elizabeth II resides; St. Paul's Cathedral designed by Christopher Wren; and Westminster Abbey, the site of royal coronations.