Harry's Walls are the remains of an unfinished artillery fort, started in 1551 by the government of Edward VI to defend the island of St Mary's in the Isles of Scilly. Constructed to defend the harbour of Hugh Town from possible French attack, the fortification incorporated Italianate-style bastions with protective orillons and would have been the most advanced design in the kingdom at the time. It was not completed, probably due to a shortage of funds and the passing of the invasion threat, and only the south-west side remains. In the 21st century, Harry's Walls are 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.