A landing at Harlingen harbour seemed likely, so the city was surrounded on the land side with Luftlande-hindernisse (obstacles to prevent aircraft landings), Panzer-sperren (tank walls), and minefields. The railway line could also be closed if needed 

For "protection" on the coastal side, structures such as Vorstrand Hindernisse were used. These consisted of three-legged concrete and iron constructions with an iron spiral tip. These tripods were placed in the sea, protruding above the water at low tide but invisible at high tide. The idea was that, in the event of an invasion, Allied landing boats would get stuck on them. 

There was a storage site for these tripods in the harbour at this very location. 

Harlingen residents, despite the warning signs, did not seem very impressed by the minefields. This prompted the Mayor, from the Nationaal-Socialistische Beweging in Nederland (Dutch National Socialist Movement), to issue posters throughout the city, warning against entering these areas and threatening imprisonment for violations. 

Harlingen was also regularly sealed off by the German forces during the war, for example, when they expected a landing or paratroopers