Zennor Quoit is a ruined megalithic burial chamber or dolmen, located on a moor about a mile (1.6 km) east of the village of Zennor, Cornwall. It dates to 2500–1500 BC. Aside from the 12.5-tonne roof, which collapsed some time between 1770 and 1865, the chamber is in good condition.
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.