Military fortress located on the south bank of the Ferrol estuary, guarding its entrance along with the Castelo de Nosa Señora da Palma (on the opposite bank, in Mugardos) and the ruins of Castelo de San Martiño. A chain was stretched between San Felipe and San Martiño to prevent the entry of enemy ships. The 18th-century structure is a representative example of the great "bastioned batteries" of the Enlightenment, with a design based on Vauban's teachings and adapted to the terrain. It has a hornwork (outer work), moat, and main gate designed by Montaigú. In the 19th century, a water-level battery was added on the sea front. It served as a prison during the Spanish Civil War. It offers magnificent views over the estuary.