Windsor Castle, England

Windsor Castle is the oldest and largest inhabited castle in the world, founded by William the Conqueror in the 1070s and home to 40 monarchs over 950 years. Its 484 acres encompass St George's Chapel, housing tombs of 10 monarchs including Henry VIII. As a working royal residence, it receives 1.2 million paying visitors annually—fewer than #5 Alhambra's 2.7 million, but its continuous occupation and 950-year legacy dwarf any other castle on this list.