The defence of the Netherlands was based on a couple of forts and flooding the country inbetween. When the Germans invaded in 1940 it failed because the Germans had airplanes.

Info about this fort: www.hollandsewaterlinie.nl/index.asp?id=118&fortid=45

Some more info in Dutch: www.fort-everdingen.nl/index2.htm
For pictures, click on "Foto's en plattegronden" on the left.

For information about the Dutch defence system (in English): www.hollandsewaterlinie.nl/index.asp?id=19&lang=en