The Castle of San Felipe sits on the north shore of the Ría de Ferrol at the narrowest point of the estuary, where the channel pinches to barely 350 metres across. It was built in the late sixteenth century under Philip II as the main defensive work guarding the approach to the future arsenal — paired with the smaller Castle of La Palma on the opposite bank and once linked across the channel by a heavy boom chain that could be hauled up to block enemy ships. The chain is gone, but the geometry of the defence is still legible from the walls.
The fort was modernised repeatedly into the nineteenth century and is now a pleasant, low-key visit: star-shaped bastions, a deep dry moat carved out of the rock, vaulted galleries you can walk through, and an upper terrace that gives the best panorama of the ría — looking across to La Palma and Mugardos and back toward the cranes of the Ferrol shipyards. There is a small museum room covering the castle's history. To reach it, drive about 12 kilometres west of Ferrol along the coast road through Brión; the last stretch is signposted "Castelo de San Filipe."