While  Buckingham Palace  is designated as the  Monarch's official residence , the current Queen,  Elizabeth II , usually  spends her weekends in Windsor Castle . Built by  William the Conqueror   (the same guy who built the Tower of London)  in  1080 , the Castle is the  longest-occupied palace in all of Europe   whose purpose was to guard the Western approach to London with its strategically important location along the River Thames. * To get to Windsor, one has to take a  30-minute train  ride to the  county of Berkshire  via the  London Paddington Station . Thanks to the  London Pass , we didn't have to worry about any lines or payment! Paddington is also the line servicing the cities of  Bristol, Cardiff, Swansea, Oxford and Worcester , thus making this a very busy place for travelers.