Fort Prins Frederik in Goeree-Overflakkee

You're standing at Fort Prins Frederik, just outside the village of Ooltgensplaat. The fort sits on its own little island beside a channel called the Hellegat. Look across the water. On the far side, near Willemstad, stood a twin fort, Fort De Ruijter. Together they had one job: stop enemy ships sailing from the Volkerak into the Hollandsch Diep.