german casemate n4 bunker from the second world war at museum and the merville battery site normandy

german casemate wwii n4 bunker detail merville battery museum normandy military defense war history vintage fortification concrete structure architecture strategic protection fortified hidden secret access entrance exit damp dark echo cold narrow long deep secure safety refuge survival escape evacuation emergency reinforced camouflage dugout earthwork excavation stronghold observation post gun emplacement pillbox atlantic wall