Once upon a time planes and heavy artillery didn't exist. Wars were fought with fists, swords, spears, muskets and... water!
The new Dutch inundation zone was built between 1815 and 1940 to protect the main cities in the west of The Netherlands. During an attack, men could flood the defense zone within 1 or 2 days. The water was too deep to walk or to drive, but to shallow to sail.
The sawn bunker near Culemborg is part of this inundation zone but was never used during war. Immediately after the completion of the defense zone it became obsolete due to modern planes and the reach of modern weapons.
Nowadays it's being used to protect the area from high water. After periods of heavy rain, water is stored in the basin next to the bunker.