Flakfortet, meaning sand-shoal fortress, is a sea fort located on the artificially built island of Saltholmreb. Flakfortet was the last of three artificial islands that the Danes created to defend the entrance to Copenhagen's harbor (the other two are Trekroner Fort and Middelgrundsfortet). Eventually Flakfortet became more of a site for anti-aircraft weapons than for coastal artillery. Its military functions ended in 1968.
The fort was built in the years 1910–1914 as part of Copenhagen's sea-fortifications. The island is 23 meters high and covers 32,000 m2. 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, thought the last was cast at Flakfortet itself.
Flakfortet is now used for recreational purposes such as concerts, and the site has a restaurant and yacht moorings. During the summer, a ferry company, Spar Shipping, has five scheduled trips per day to the island, with the trip from Nyhavn taking about 40 minutes each way. There is an overnight hostel on the island too.
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