Extending 135 km around the city of Amsterdam, this defence line (built between 1883 and 1920) is the only example of a fortification based on the principle of controlling the waters. Since the 16th century, the people of the Netherlands have used their expert knowledge of hydraulic engineering for defence purposes. The centre of the country was protected by a network of 45 armed forts, acting in concert with temporary flooding from polders and an intricate system of canals and locks.