Early in the Eighty Years' War of Independence against Spain the Dutch had realized that flooding low lying areas formed an excellent defence against enemy troops, as was demonstrated, for example, during the siege of Leiden, 1574. In the latter half of the war when the economic heartland of the Dutch Republic (i.e. the province of Holland) had been freed of Spanish troops. Maurice of Orange Nassau planned to protect it with a line of flooded land protected by fortresses that ran from the Zuiderzee (present IJsselmeer) down to the river Waal.
In 1629 prince Frederick Henry started the execution of the plan. Sluices were constructed in dikes and forts and fortified towns were created at strategic points along the line with guns covering especially the dikes that traversed the water line. The water level in the flooded areas was carefully maintained to a level deep enough to make an advance on foot precarious and shallow enough to rule out effective use of boats (other than the flat bottomed gun barges used by the Dutch defenders).
Under the water level additional obstacles like ditches, trous de loup and later barbed wire and mines were hidden. The trees lining the dikes that formed the only roads through the line, could be turned into abatis in time of war. In wintertime the water level could be manipulated to weaken ice covering, while the ice itself could be used when broken up, to form further obstacles that would expose advancing troops longer to fire from the defenders.