This construction, which began in 1747, was completed in 1774 with a design by Sánchez Aguilera. The Fontelonga sea door is the only one remaining from the wall which surrounded Ferrol square. Today it no longer serves its original purpose due to land reclamation from the estuary which was carried out to expand the shipyard. This construction is in fact an ensemble of various elements: the defensive wall, with a high door and broken lines along its flanks, the low door with a jack arch and lookout post which gave access to the old fountain and a sloping corridor with steps and a central ramp. A large flying buttress which supports the sloping wall has also been preserved. Nearby, we find the remains of the San Antonio door, one of the oldest doors which gave access to the shipyard.