The Tower of London

Construction of Her Majesty's Royal Palace and Fortress, more commonly known as the Tower of London, began in 1078.  The iconic White Tower is in the centre of the fortress and has withstood many attacks through the centurys, none of them successful.  You'll note that the main entrance is elevated by a little more than one storey; if the invaders some how got past the hail of arrows, burning balls of oil soaked reeds, boiling oil and what have you en route to the White Tower steps, made of wood, would be set on fire thereby making access to the only entrance almost impossible.