Windsor Castle, dating to the 11th century, has long been associated with the English and British royal families. Since Henry I, who ruled from 1100-1135, reigning monarchs have used the castle, making it the longest-occupied palace in Europe. The castle’s original purpose was to protect Norman dominance around London’s outskirts. It also oversaw a strategically important part of the River Thames. Royals have used Windsor Castle for refuge, including during World War II’s Luftwaffe bombing campaigns. Today it is a favorite weekend retreat for Queen Elizabeth II. The castle sits on 13 acres and is accessible from London by train.