The Defence Line of Amsterdam (Dutch: Stelling van Amsterdam) is a 135-kilometre fortification line around Amsterdam, which would function as a national redoubt. It comprises 45 forts, as well as dams, dikes, locks, pumping stations, batteries and casemates. The line could be inundated in time of war. The inundation was designed to give a depth of about 30 centimetres, too little for boats to cross.

At Fort Aalsmeer lie the remains of a lock complex that was specially constructed for the defence line. The sluices were built in 1891 and the double lift-lock is the largest of the sluices built for the line.