The Calahorra tower (Torre de la Calahorra) is a fortified gate in the historic centre of Córdoba. The edifice is of Islamic origin. It was first erected by the Almohad Caliphate to protect the nearby Roman Bridge on the Guadalquivir. The tower, standing on the left bank of the river, originally consisted of an arched gate between two. A third tower was added to the existing ones, in the shape of two cylinder connecting them.
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.