Dover Castle lies at what has been the most strategically important part of England since ancient times. Dover is at the shortest point between England and the continent (21 miles to Cap Gris Nez in France). It has been the focal point of this country’s defensive and offensive operations for centuries. There has been a fort at Dover almost certainly since Iron Age times. It is on its escarpment that the present castle is built. The mighty keep and walls were constructed on the orders of King Henry II (1154-89). It has uniquely a concentric set of fortifications with an initial set of walls directly surrounding the keep itself and an outer set of walls providing much needed additional protection from a besieging army. The castle was subject to a major siege in 1216 when an invading French army besieged the castle and breached the outer walls of the defences. The defending garrison was able to hold out, but the outer fortifications of the castle had to be rebuilt and considerably strengthened following the siege. Whilst the castle endured no further action of note during medieval times it never fell to rack and ruin. Given its vitally important location its defences were always brought up to date and strengthened most notably in the 18th century and at the beginning of the 19th century during the Napoleonic wars when vast underground tunnels were constructed in the cliffs at the southern end of the castle. These tunnels were turned into a wartime naval HQ during World War Two (1939-1945). It was from this HQ that one of the most remarkable episodes of the war was conducted, Operation Dynamo, the emergency evacuation of over 330,000 British, French and Belgian troops from the beaches of Dunkirk in May-June 1940 under the direction of Rear-Admiral Bertram Ramsay.