For many centuries water is the biggest way of transporting goods for trade between The Netherlands and other countries.
Hellevoetsluis is one of the many small towns with his own harbor for this purpose. But Hellevoetsluis once also was a navy harbor. Almost all harbors that are at the coast of the North Sea have locks of the same kind that Hellevoetsluis has to maintain a high water level in the harbors at low tide. All locks also have a function to protect the inland from incoming salt water when the level of the sea water is at high risk level at times of storm and extra high tides.
Nowadays ships with trading goods and navy vessels are no longer passing the lock of Hellevoetsluis, just small boats for leisure are now entering the lock chamber. So once their main function was for trade and safety and now their main function is to protect the inland against salt water.