In 1922 the Defence Line of Amsterdam was included in what is referred to as the Vesting Holland (Holland Stronghold). This area was to be defended tenaciously in times of war. It comprised the province of
Noord-Holland south of the line from Edam to Castricum, the province of Zuid-Holland, and the western half of the province of Utrecht. As a result, the northern and eastern parts of the Defence Line of Amsterdam were maintained as a defence line, but its function as a circular defence line became defunct. The other forts remained military objects within the Vesting Holland as accommodation for staff and material.

Throughout the 1930s, particularly with the growing threat of war in Europe, the Vesting Holland was reinforced in several ways; for instance, concrete bunkers were added to the eastern part of the Defence Line of Amsterdam and inundations remained an important element in defending the country. Just before the outbreak of World War II, the Dutch government decided on a general mobilisation (August 1939) and proceeded to partially inundate the land at the eastern front of the Vesting Holland; this coincided with the New Dutch Inundation Line.

It was only during the German offensive in May 1940, when the Germans threatened to cross over the IJsselmeer from Friesland, that the northern front of the Vesting Holland (this was the northern part of the Defence Line of Amsterdam) was put in a state of defence. For the first time in its history, inundation was actually carried out in this part of the Defence Line. Yet it was clear that the former Defence Line of Amsterdam had definitively lost its military value through modern warfare, like the massive use of planes and airborne troops. During the occupation (1940-1945), the armoured gun turrets were removed from most of the forts and melted down for the German war industry. The fort at Spijkerboor in the Beemster Polder is the only fort of the Defence Line of Amsterdam that still has an armoured turret with two canons, which can be visited by the public.

Main entrance of the Fort at Edam