The impressive, Gothic-styled Houses of Parliament occupy the original site of the Palace of Westminster. Combined with their famous clock tower (commonly known as Big Ben), they're synonymous with London and a necessary stop. Although the building's exteriors are spectacular, it's even more interesting to head inside and see the House in action from the Visitor's Gallery – particularly if you go when the Prime Minister is subjected to Question Time, when disorder is common. Overseas visitors may tour Parliament only during Summer Opening in August and September, but may attend debates whilst the parliament is in session too. TUBE: Westminster