I once went to visit the extensive underground tunnel system dug into the 'White Cliffs' of Dover under the castle.. which were the combined operational command centre of the Army Navy and Airforce during the second world war from which the evacuation of Dunkirk was overseen.... They've been carefully restored and opened to the public. Short vid here if anyone wants a look..
Dover Castle - Kent TV
It is ingenious the way everything was organised.. including a fully equipt hospital, and a complete telephone and communications system, that allowed them to keep in contact with bases all over the country... couldn't imagine living there for any long period of time thouogh... it was clustrophobic after an hour or so....