The Defence Line of Amsterdam was built between 1883 and 1920, and extends 135Km around the city. It consists of 36 forts, 2 coastal forts, 2 fortressess, 3 batteries and 2 coastal batteries, and several flooded areas known as inundations. It is the only example of a fortification based on the principle of controlling water, and is a testament to the Dutch knowled of hydraulic engineering. There are several cycle routes through the defence line, and many of the forts open for viewing on Monuments Day. The map location points to an old depot that was part of the line.