Castillo de Santa Cruz, Military castle on Santa Cruz Island, Oleiros, Spain.

Castillo de Santa Cruz stands on the island of Santa Cruz in Porto de Santa Cruz, Oleiros. Its square light-grey masonry tower has a flat crenellated top, a small arched window and a taller round turret at one corner; a lower wall links it to a second tower behind trees. A metal footbridge on wooden trestles crosses in front of the lower stone base wall, and a long low sea wall runs to the right.

Construction began in 1594 or 1595 under Diego das Mariñas, captain general of Galicia, to plans by military engineer Pedro Rodríguez Muñiz. In 1639 the castle took part in repelling the Franco-Dutch fleet of the archbishop of Bordeaux, Henri d'Escobleau de Sourdis. Between 1784 and 1792 new bastions, pavilions and parapets were added under Martín Cermeño.

The fortress now serves as a center for environmental education where visitors learn about marine life and coastal ecosystems. The exhibitions show how local people interact with the sea and work to protect these natural spaces.

To visit the fortress, cross the stone bridge from the mainland in Oleiros, which is easy to reach on foot. Guided tours are available several times per week and provide context about the site's history and current use.

The castle stands on the islet of Santa Cruz, cut off at high tide; Pascual Madoz noted in 1847 that it was then a battery of eight guns on slate and earth that the sea undermined. Its small size is often missed: Jerónimo del Hoyo counted four pieces of artillery there in 1607.