In the nineteen thirties, the Dutch built several permanent defensive structures at both ends of the Afsluitdijk to protect the levee in case of hostilities and to prevent it from falling into enemy hands. The works at Den Oever and Kornwerderzand included artillery casemates, road blocks and emplacements for small caliber anti-aircraft guns. During the German attack on the Netherlands of May 10th, 1940, a platoon advanced on the Afsluitdijk from the northeast, via Groningen and Sneek. It took the Germans several days to secure the levee. After the Dutch surrender, the Germans took over the Dutch casemates and added several of their own bunkers. Kornwerderzand became part of Stützpunktgruppe Harlingen. The area was defended by three additional bunkers: two types 667 (a Kleinstschartenstand) and one type 612. The 667s were oriented towards the south and covered the north and south side of the levee; the 612 aimed west over the Waddenzee. The garrison of WN27a was made up of a ”Deichkompanie” [Deich = levee] and a contingent of reinforced border patrols [Verstärkter Grenzaufsichtsdienst] totaling about 56 men. The complex included reused Dutch casemates, gun bunkers, a few thin-walled buildings such as a kitchen, light AA-gun emplacements and several Tobruks. All these bunkers are still present and they are in fair condition. The Kazemattenmuseum Kornwerderzand occupies a number of them.