The Castle of San Marcos is a church-fortress built on the foundations of a 10th century mosque, located in El Puerto de Santa María, Cádiz, Spain, which was erected by the monarch Alfonso X El Sabio in honour of the Virgin after the capture of the city.

The Castle of San Marcos in El Puerto de Santa María was built from an old Islamic mosque that was transformed into a fortified church in the time of Alfonso X the Wise in around 1264, who reinforced its foundations using the remains of a nearby Roman building, providing it with vaulted roofs and fortifying the sanctuary with four towers, two hexagonal and the other two quadrangular, the ground floor of the former being used as the main chapel for the Virgen de los Milagros (Virgin of Miracles).

The wall of the quibla where the mihrab was located and the Almohad appearance of the building are still preserved, as can be seen in the horizontal, smooth lines with a raised edge at the top of the hexagonal tower and in its schiffed vaults. King Alfonso X established his new military order of Santa María de España in this church, which soon disappeared after being merged with the Order of Santiago.

As a castle it served as a defence against new invasions from North Africa, and later Sancho IV would donate it, along with the town, to the Genoese admiral Benedetto Zaccaria, and later to Alonso Pérez de Guzmán El Bueno, who in turn donated it to his daughter Leonor de Guzmán in 1306 on the occasion of her marriage to Luis de España, thus giving rise to the House of Medinaceli, lords of the town in the time of Isabel de la Cerda y Guzmán.

Juan de la Cosa's map or chart is a map painted on parchment, 93 cm high by 183 cm wide, which is kept in the Naval Museum in Madrid. An inscription states that it was made by the sailor Juan de la Cosa in 1500 in Puerto de Santa María (Cádiz). Its rich decoration indicates that it was probably commissioned by a powerful member of the court of the Catholic Monarchs. This personage and map has a small corner next to the Castle of San Marcos where you can see a bust of Juan de la Cosa and a replica of the map in a small fountain.