Dover Castle

Henry II spent the 1180s turning this clifftop into what has been called the most advanced castle design in Europe: a great tower that doubled as a palace, wrapped in walls with projecting towers that widened the defenders' field of fire. The design was tested in 1216, when Prince Louis of France besieged it during the war between King John and his barons. The northern barbican and main gate were badly damaged, a reminder that the entrance was always a castle's weakest point, but the garrison held. Dover also keeps going where most castles stop: tunnels beneath let later garrisons live and work under cover, so one visit spans medieval stone and modern wartime defense.