This is the most important military construction in Ferrol. Located opposite the Castle of La Palma (in Mugardos) and the now-disappeared Castle of San Martiño, the three fortifications made the Ferrol Estuary impregnable, forming a ‘triangle of fire’ that no enemy power of Spain dared to attack. At night, the castles of San Felipe and La Palma were connected by a chain, thus preventing the passage of enemy ships. Admiral Pitt is attributed with the phrase that if Great Britain had an estuary like this, with its castles, they would protect it with a wall of silver. The construction of the castle began in 1557, sponsored by Philip II, from whom it takes its name. However, the building we preserve today is from the 18th century, when it was remodeled. The major works of the castle were finally completed in 1775. In the 18th century, San Felipe was considered an example of a ‘bastioned battery.’ This design featured, on one hand, a zone of cannon batteries directed towards the sea, and on the other hand, it had a system of bastions – or protruding walls – covering the flanks of the curtains (the frontal walls). These bastions also served to repel land attacks. San Felipe, in a constant process of restoration, surprises visitors both for its construction scheme and for the beautiful views of the estuary that can be obtained from its location.