The Puerta de Palmas is a former city gate built in 1551. It has two cylindrical towers flanking the entrance door. Prince Philip II and Emperor Charles V and date of construction are mentioned on the outer side of the tower. The towers are fortified with battlements and they have two decorative cords at the top and bottom levels. Its entrance is east-facing, and is double-arched and is decorated with medallions of the shield of the Emperor Charles V. It was once used as a prison, but has since undergone many renovations and has been an entrance point to the city.
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.