In the 12th and 13th century, Näs (or Visingö) Castle on the southern end of Visingsö was the residence for the fragile Swedish monarchy. It was built probably by Sverker I of Sweden (died in 1156) or his son Charles VII of Sweden (Karl Sverkersson). Näs was thereby the oldest Royal castle in Sweden. It was a residence 5-6 kings before in 1318 Birger I (Birger Magnusson) pawned it to Danish. The castle was probably demolished soon after.
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.