There are two ancient hill forts in Porvoo, so-called small and big one. There is burial ground in a small hill from the Roman Iron Age (0-400 AD). The items found in excavations reveal that Porvoo river has been a remarkable trading centre already in prehistoric times and local people has had connections to Estonia and Latvia.
The bigger hill fort is one of the largest in Finland. It was used for defensive purposes already in the Viking Age (800-900 AD), but the fortifications date from the late 14th century. Today remains of double walls and dry moat are visible and restored.
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.