Kastellet is a small citadel located on the islet Kastellholmen. The first fortification on the location was built in 1667 designed by Erik Dahlbergh. It exploded in June 1845 and subsequently a new was built 1846-1848 to the design of architect Fredrik Blom. It consists of a round tower with red brick walls and a 20 meter high stair tower. On the top flies the Military Ensign of Sweden, it is hoisted and lowered every day, indicating the nation is at peace.
On May 17, 1996, the Norwegian Constitution Day, some Norwegian expats raised the Norwegian flag in the tower. Though such an action would historically have been taken as a declaration of war, a diplomatic crisis could be avoided.
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.