The fort was built in the years 1910–1916 as part of Copenhagen's sea-fortifications. The island is high and covers . Construction of the artificial island required the use of
caissons of reinforced concrete, possibly the first use of such structures. The caissons were first cast at Lynetten quai on
Refshale Island, though the last was cast at Flakfortet itself. The fortifications extended over two floors and included a barracks that once housed the garrison of 550 men. Initially, the fort's armament consisted of four 29 cm M1910 howitzers, six 21 cm M1913 guns, and four 75mm
quick-firing guns. Later, it received two 47mm
ballonkanoner (anti-balloon guns), another 75mm, and 8mm machine guns. It also received one 90 cm and four 150mm searchlights.
Marshall Plan aid enabled the Danes in 1950 to install a variety of anti-aircraft guns. These included four heavy German 105 mm guns, which were probably moved from Hansted (
Hanstholm), six twin-40mm guns (American copies of the
Bofors M36, and some 20mm anti-aircraft guns. These newer weapons replaced some of the earlier artillery. Flakfortet then remained in military use until 1957. In 1965, the Danes decided to establish a
Hawk anti-aircraft missile battery on Middelgrundsfortet. However, this fort required extensive renovation so in the interim (1965-1968), the Danes temporarily placed the HAWK battery on Flakfortet. For almost a decade thereafter the fort again stood unused. In 1975, the Danish
Ministry of Defence leased Flakfortet to the Copenhagen Sailing Association and opened the site to the public. Unfortunately, one result was that during this period the fort was the victim of extensive vandalism. The Danish Ministry of Defence owned Flakfortet until 2001. It was put up for sale in 1999 under the guidance of
Statens Ejendomssalg (commonly known as
Freja), which is the department under the Danish
Ministry of Finance responsible for selling surplus state-owned property. In June 2001 the Swedish company
Malmökranen purchased Flakfortet. ==References==