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.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.