The Roman Amphitheater in Trier is designated as part of the Roman Monuments, Cathedral of St. Peter and Church of Our Lady in Trier UNESCO World Heritage Site. The arena, built in the 2nd century A.D. for cruel games with gladiators and animals, had a seating capacity of about 20,000. When you enter the premises you walk through the ruins of the entrance gate. This was used as a quarry in the Middle Ages. The arena itself is surrounded by a protecting wall with openings for animal cages. Underneath the arena is a vast cellar where, in Roman times, prisoners sentenced to death were kept alongside exotic wild animals like African lions or Asian tigers.
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.