Oulu castle (Uleåborg) was built in 1590 for a stronghold to Swedish soldiers on their way to fight against Russian Karelia. The castle was mostly made of wood and earth walls. There probably was an earlier medieval castle on the same location. The Russian Sophia Chronicle has recorded that men from Novgorod tried to conquer a new castle in the Oulu River delta in 1377 but were unsuccessful. King of Sweden, Charles IX ordered to rebuild the castle in 1605. Old wooden parts were demolished and a new wall was built around the castle island.
Russians burned down wooden parts of the Oulu castle during the Great Northern War in 1715. Final destruction occured in 1793, when thunderstorm set the castle on fire and gunpowder magazine exploded.
Wooden constructions on the remaining powder magazine date from 1875 when the Oulu School of Sea Captains built their observatory on the site. The building has been a cafeteria since 1912 with a small exhibition on the castle history.
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.