Castle of San Marcos (also Castillo de Alfonso X El Sabio) is a medieval castle located in El Puerto de Santa María, Cádiz, Spain. The castle was erected as a fortified church by King Alfonso X of Castile. It was built on the site of a mosque of which the wall of the qibla survives.

Close to the castle there is a replica of the map of Juan de la Cosa, along with an explanatory plaque, and a little fountain.

The castle was declared a national monument on 30 August 1920 and is currently listed in the Spanish heritage register as a Bien de Interés Cultural.

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  Lateral view
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  Detail of tower
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  Statue of Alfonso X of Castile
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  Far view
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  Paso in a procession of Holy Week

• List of Bienes de Interés Cultural in Cádiz