The Fort in Velsen forms a part of the Defence Line which extends 135KM around Amsterdam. The Defence Line was constructed between 1883 and 1920, and is the only example of a fortification system which works by controlling the flow of the water. The defence line is designed to keep water at bay in times of peace, and flood strategic points just enough to make them impassable on foot (but not enough to make boats usable) during times of war.
