On the south side of Polder Beemster are a few fortified forts of the Defense Line of Amsterdam. The earthworks were constructed in 1886-89, the defendable earthworks in 1895. The bomb-free buildings of concrete were realized in 1911-'14 in the form of a main building and a lift-dome building, interconnected via a barrel. The head support of this part of the position was the forem. fort at Spijkerboor (Westdijk 46), which served as the conclusion and defense of the Beemsterringvaart.
It has a two-storey main building (1911) and a very heavy armored dome. There are also a fort guard house and a shooting range (circa 1950). After 1918 it served successively as the House of Detention for soldiers, as an internment camp and as a camp for conscientious objectors. Some wall paintings from around 1920 have been preserved in the fort. From the form. fort north of Purmerend (Kwadijkerweg 2-8) the earthwork from 1886 was converted into a defensible earthwork in 1895 to defend the Beemsterringvaart and a few roads. There are bombproof buildings (1912) and a fort guard house (Kwadijkerweg 2). The fort is used as a wine warehouse. The form. Fort on the Nekkerweg (near Nekkerweg 24) served as the conclusion and defense of the Nekkerweg and the Volgerweg. It has bombproof buildings (1911) and a wooden fort guard house (Nekkerweg 24). In 1914, the bomb-free buildings were completed from the forem. fort on the Middenweg (at Zuiddijk 13) - with plastered fort guardian house (Zuiddijk 13) - and the forem. fort at the Jisperweg (at Zuiddijk 4).
Other parts of the position in the Beemster are an inundation sluice (at Zuiddijk 14, 1890-'91), a dam lock with arch bridge and double shot bar rebel (at Rijperweg 99, 1899), and a dam lock at the Jispersloot (Volgerweg approx., 1914) .
