Arcos de la Frontera Castle was first constructed in the 11th century by the Moors when Arcos was briefly a taifa (small kingdom) before being captured by the Christians in 1250. The Christians rebuilt the castle as part of their campaign to retain their hold on the town, which was in a strategically important position for maintaining the border between the Muslim and Christian kingdoms.
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.