Porte Mars is an ancient Roman triumphal arch in Reims. It dates from the third century AD, and was the widest arch in the Roman world.
The arch stands 32 metres long and 13 metres high. It was named after a nearby temple to Mars. The arch has many highly detailed carvings on its exterior and on the ceilings of its three passageways. Local folklore says that the inhabitants of Rheims built the arch in gratitude when the Romans brought major roads through their city. It served as a part of castle of archeveque and a city gate until 1544 was closed of it. In 1817, the buildings around it were removed, bringing the arch into full view.
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.