Built by Vauban between 1667 and 1672, the purpose of Citadel of Arras was to protect the city from the attacks (Spanish troops coming from the Netherlands). It has been nicknamed La belle inutile (the beautiful useless one) by residents as it has never been directly involved in heavy fighting and didn't keep the Germans form occupying the city in either Word War. Since 7 July 2008 it is part of the UNESCO World Heritage Sites Fortifications of Vauban which includes eleven other fortifications. Within the citadel on the side of La Place de Manœuvre a small Baroque-style chapel was built. Outside, Le Mur des Fusillés (the wall of the people executed by a firing squad) pays tribute to the 218 members of the French Resistance shot in the citadel's ditch during World War II.
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.