Torsburgen was the largest ancient fortress in Northern Europe. It was originally constructed at the beginning of the 1st century AD. It was reinforced during the 4th century and used until c. 1100 AD. A timber-laced stone rampart encircles an area of 12 hectares. Scholars estimate that nearly 1000 soldiers would have been needed to defend it and it could have been providing refuge to the entire population of Gotland that is calculated to be between 6,000 and 10,000 in the early medieval period.
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.