Jerez de los Caballeros Castle dates back to the 13th century. The castle stands on the western part of a hill, around which the town of the same name extends. Since the Neolithic it was inhabited by Phoenicians, Celtic Betures, who were initially known as 'Ugultiniacum', Romans, Visigoths, Muslims and chivalric orders such as the Temple and Santiago settled chronologically in these territories.
The current castle was built by the Templars, who refused to lay down arms when the order was suppressed, and came to a sticky end. You can wander around at will, but it's basically just the impressive walls that are preserved, with pretty gardens dotted around.
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.