Cromwell's Castle is an artillery fort overlooking New Grimsby harbour on the island of Tresco in the Isles of Scilly. It comprises a tall, circular gun tower and an adjacent gun platform, and was designed to prevent enemy naval vessels from entering the harbour. The castle was built in two phases; Sir Robert Blake constructed the tower between 1651 and 1652 in the aftermath of the Parliamentary invasion of the islands at the end of the English Civil War, and Master Gunner Abraham Tovey added the gun platform during the War of Jenkins' Ear around 1739. The tower fell into disuse soon afterwards, and in the 21st century is managed by English Heritage and open to visitors.
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.