Situated on the little island, the defensive Sancti Petri fortification was once one of a series of forts that protected the inlet, Caño de Sancti Petri. Of irregular shape and in the Moorish style, it dates from the 13th century. During the 1st century, the Phoenicians settled on the island, founding a temple dedicated to Melqart, their god. When the Romans arrived they dedicated the temple to Hercules.
The castle's watchtower is the oldest building, while the walls and the interior date from the 18th century. The castle was in an advanced state of deterioration, but the authorities of the municipalities of San Fernando and Chiclana de la Frontera funded a major rehabilitation programme.
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.