   the unesco world heritage site known as the defence line of amsterdam (in dutch named stelling van amsterdam) is a 135&nbsp;km long ring of fortifications around amsterdam, consisting of 42 forts located between 10 to 15 kilometers from the centre, and lowlands that can easily be flooded in time of war. the flooding was designed to give a depth of about 30&nbsp;cm, insufficient for boats to traverse. any buildings within 1&nbsp;km of the line had to be made of wood, so that they could be burnt and the obstruction removed. the stelling van amsterdam was constructed between 1880 and 1920. the invention of the aeroplane and tank made the forts obsolete almost as soon as they were finished. many of the forts now are under the control of the town councils and the nature department, and may be visited. monuments day, on the second saturday in september, is the ideal day to visit as entrance is free. ==function== the stelling van amsterdam was primarily a defensive "water line" (in dutch: waterlinie). in the event of an enemy attack, large tracts of land around amsterdam would be inundated with water, preventing the enemy from advancing. amsterdam would function as a national redoubt or reduit, as the last stronghold of the netherlands. forts were built where roads, railways, or dikes crossed through the water line. at such locations there would be no water to stop the enemy, and therefore the forts were intended to shell the enemy. ==construction== the law for the construction of the stelling van amsterdam was passed in 1874. during the preparations prior to construction, it became apparent that the design was already outdated by modern technical advances. the invention of the brisance grenade (which would explode on impact of the target) necessitated the change from masonry to concrete forts. but the dutch did not have the required experience yet using and building with concrete. therefore extensive tests had to be performed where concrete structures were shelled with the heaviest artillery available at that time. further delays resulted from the fact that the sand f
Buildings_and_structures_in_North_Holland Forts_in_the_Netherlands History_of_Amsterdam Stelling_van_Amsterdam Visitor_attractions_in_North_Holland World_Heritage_Sites_in_the_Netherlands