One of the method of defence invented by the Dutch during the centuries, to make up for the paucity of natural defence features, was a system of forts, sluices and canals built in the 17th Century in the Eastern part of the country, that would have allowed the area to be flooded in case of enemy attack.
It obviously never served the Dutch very well, however it left behind an interesting system of forts, many of them open to the public, that make for an interesting day out. I have visited several near Utrecht and Amsterdam.
Next on my to-do list is the Fort de Roovere, near Halsteren in West Brabant which has only recently been connected to the mainland, thanks to pretty impressive sunken bridge: the Moses Bridge.
Fitting name, since it's almost invisibly parting the water...