Middelgrundsfortet is a sea fort located in the Øresund between Copenhagen and Malmö. It was constructed 1890–1894 as a part of Copenhagen's sea-fortifications, partly from material excavated from Frihavnen. It is one of three artificial islands that were created to defend the entrance to Copenhagen's harbor (the other two are Flakfortet and Trekroner Fort).
Middelgrundsfortet was the largest sea fortress in the world, and is still the largest man-made island without abutment, with an area, including wave breakers, of approximately 70 000 m²; the buildings total approximately 15 000 m². A HAWK battery was placed on the island. Until 1984 the fortress was a military area; in 2002 it was sold to a private investor. Ferries connect Middelgrundsfortet with Langelinie, Copenhagen.
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.