St Catherine's Castle is a Henrician castle in Cornwall, England, built by Thomas Treffry between approximately 1538 and 1540, in response to fears of an invasion of England by France and the Holy Roman Empire. The D-shaped, stone fortification, equipped with five gun-ports for cannon, overlooked the mouth of the River Fowey in Cornwall. It was protected by a curtain wall and the surrounding cliffs.
The castle remained in use for many years until it was closed at the end of the Napoleonic Wars in 1815. Brought back into service in 1855 during the Crimean War, it was fitted with two new artillery positions, but it soon became obsolete and was abandoned. During the Second World War the castle was refortified and used to house a battery of naval guns, protecting the coast against the threat of German attack. At the end of the conflict the castle was restored to its previous condition and is now managed by English Heritage as a tourist attraction.
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.