42 Forts encircling Amsterdam to provide defense

The Defense Line of Amsterdam, was built between 1883 and 1920. This 135 km long ring of forts is the only example of a fortification based on control of water in history. The network of forts and artillery worked together with flooding of lowlands and a system of canals and locks.

The defense line was a good idea designed to guard the capital city. Unfortunately the invention of airplanes and tanks quickly made the forts obsolete. Only some of these forts that make up the Defense Line of Amsterdam (Stelling van Amsterdam) are open to the public.