Coetan Arthur dolmen, also known as Arthur's Quoit (not to be confused with Carreg Coetan Arthur, near Newport) is the remains of a Neolithic burial chamber (also known as a quoit). It dates from around 3000 BCE. The site, situated on the hillside close to St Davids Head in Pembrokeshire, Wales, is the collapsed chamber of what is presumed to be a passage grave which also has a round barrow. The massive capstone measures approximately 6 metres by 2.5 metres and is supported on one side by an orthostat approximately 1.5 metres in height.
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.