The forts in the Defence Line of Amsterdam are of various styles, depending on the function of the defence work. They were not all built at the same time either. Constant changes had to be made to both the existing forts and the designs of the planned forts in accordance with new developments in artillery techniques.

The first newly constructed fort of the circular defence line was located by the town of Abcoude and was completed in 1885. It consisted of a ‘bomb-free’ barracks, depots and open artillery emplacements between earthen ramparts and was entirely surrounded by a moat. This design was to be applied to the entire Defence Line of Amsterdam, but with the invention of the high-explosive shell around that time, the defensive value of this type of fort dropped significantly. Construction of the remaining forts was put on hold pending a new, adapted design.

Construction on the forts of the Defence Line resumed in 1897: the new type of fort was long and narrow and had built-in bunkers made of a new kind of concrete. The forts were covered with a 25-cm layer of earth so that the explosion pressure at impact would be directed outwards and not through the roof. Two bunkers were positioned at the rear in the entrance to the fort to provide support fire on the flanks and for
close-range defence. The six forts mentioned earlier, Pampus, Diemerdam, Durgerdam, IJmuiden, Velsen and Spijkerboor, were design differently because of where they were located in the Defence Line.

The Defence Line even had forts that were already outdated, like the brick tower forts in Muiden and Weesp built around the middle of the 19th century, which formed part of the New Dutch Inundation Line until 1892. An artillery battery was positioned on the top. In 1913 the old forts Hinderdam and Uitermeer were also added to the Defence Line of Amsterdam.