Piedrabuena Castle is part of defense line of Guadina and Tajo rivers near Portugal border. The oldest parts date from the late 14th and early 15th centuries. It has been head of the Encomienda of the Order of Alcántara since the 13th century.
The exterior enclosure is a quadrangular construction with cylindrical towers at the corners. In the interior there are four corridors, the keep and the prison tower. The residential function of this building is easy to see, with galleries open to the outside and a cloistered patio, all in the manner of an urban palace.
Today Piedrabuena Castle is privately owned.
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.