Castles are an integral part of the history and landscape of Britain, but the art of building a castle was brought across the channel by William the Conqueror. We visit the medieval quarry in France which supplied the stone for iconic buildings such as the Tower of London and Canterbury Cathedral. Nick Crane sets sail from Dover to visit the white cliffs of France. Connected by land before a mega flood carved the channel, Nick discovers that these divided cliffs are facing parallel challenges of coastal erosion.