A water line is a line consisting of a continuous whole of areas to be flooded and fortifications – fortresses, fortresses – on the vulnerable parts in between. We call this flooding ‘inundation’. Once the land is flooded, the enemy cannot get past it.
In the Netherlands we know the Old and the New Dutch Waterline. The Old Dutch Waterline only protected ‘Holland’, the area that we now know as North and South Holland. Due to the arrival of the New Dutch Waterline, the city of Utrecht is also protected.