The Nevenbatterij van het Fort bij Aalsmeer is a part of the Defence Line of Amsterdam. Work began on the defence line in 1880, and was completed by 1920. The defence line consisted of a network of forts, earthen batteries, pumps, and dykes, that could be used to flood strategic areas to prevent troop movement, and defend key industrial and trade points. The entire defence line is now recognised as a Unesco protected monument.
