The Grebbe Line was a forward defence line of the Dutch Waterline, based on inundation. The Grebbe Line ran from the Grebbeberg in Rhenen northwards until the IJsselmeer.
Grebbe line near Scherpenzeel
The Vestingwet or Wet tot regeling en voltooiing van het vestingstelsel was signed on 18 April 1874. The law dealt with the defense of the Netherlands against enemy attacks.
Grebbe Line
…Other Defence lines of the Netherlands (in Dutch) Grebbelinie website…
Overview of the Dutch defense lines in 1940
Stelling van het Hollands Diep en Volkerak Defense line of Hollands Diep en Volkerak
West Brabantse waterlinie West Brabentse Water line