Berlin wall Pillars of Hercules arch dam backstop bank bar barrage barrier barrier of secrecy bear-trap dam beaver dam blackout boom border border ground borderland breakwater breastwork brick wall buffer bulkhead bulwark censorship cofferdam curtain dam defense dike ditch earthwork embankment fence frontier frontier post gate gravity dam groin hush-up hydraulic-fill dam iron curtain ironbound security jam jetty leaping weir levee logjam march marches marchland milldam moat mole mound oath of secrecy official secrecy outpost outskirts pall parapet portcullis rampart repression roadblock rock-fill dam seal of secrecy seawall security shutter dam smothering stifling stone wall suppression three-mile limit twelve-mile limit veil veil of secrecy wall weir wicket dam work wraps