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 Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.