This unique building was designed by Sánchez Bort in 1765. It is a simple, vast building which at one point housed over 1,000 prisoners who were posted to the Ferrol base to carry out forced labour, bailing out the docks, a task which required a large number of workers due to its magnitude. The building still conserves the bars which prevented prisoners from escaping and also the rudimentary kitchens where the broth that the prisoners were fed was prepared.

Today, the San Campio prison houses Ferrol´s Naval Museum and Naval Library. The museum is a magnificent summary of 300 years of history of the Spanish Armada. Countless pieces over various rooms show this history through mock-ups, scale model ships, archaeological remains, target practice pieces, decorative pieces, flags etc. The library is accessed via an impressive stone staircase with a rope as a bannister and was set up in 1872. It holds a valuable collection of books and maps, especially about military science and engineering, in surroundings decorated by furniture taken from ancient ships.