So much massive
construction and moving of earth have taken place over the
centuries in Naples that it is very difficult at any point
in the city to stand back and see in your mind's eye what
it all used to look like. This is particularly true at the
port. If you stand directly at the passenger entrance of
the port, at the Maschio Angioino
castle, much of the space to the west (on your
right, as you face the sea)—that is, the naval college
next to the castle and then the Royal
Palace next to that— plus the entire layout of docks
and piers at water's edge—Molo (pier) Beverello, Molo S.
Vincenzo, and Molosiglio—are on the site of what used to
be the naval "arsenale" for the kingdom of Naples. Indeed,
all of the city blocks along the seaside road north of the
tunnel and running over to the Santa Lucia section of town
are on landfill from 1900. Much of that area, too, was
part of the "arsenale".
The Italian term "arsenale" means a weapons storehouse, as it does in English, but, in this case, the usage is in the older sense of a "naval shipyard"—the place where they built warships, and that is how I shall use the English word. The history of the arsenal at the port goes back to 1239 when Frederick II, the great Holy Roman emperor, expanded the port facilities to accommodate six galleys. Just what he was expanding is not clear. The capital of the kingdom was still in Sicily at the time, and there is not much evidence of earlier port facilities before the founding of the Kingdom of Naples in the 1100s. (A thousand years earlier, the Roman commercial port was where Piazza Municipio now is, but Roman military shipbuilding took place elsewhere, just up the coast at Baia at Portus Iulius, the home port for the Western Imperial Fleet.)
The first large-scale attempt to create a new arsenal was in 1278 when Charles I of Anjou ordered a port facility that would contain 50 galleys and be able to outfit 6-8 at a time. The facility was in place even before the "new castle", Maschio Angioino, but clearly was part of the same overall expansion of the new capital of the kingdom.
In the mid-1400s, shipbuilding led to a timber shortage, remedied by massive months-long timber fetching expeditions to the southern wilds of Calabria, Zervò mountain in Calabria near Oppido Mamertina. It was then but a short distance to the coast and the port of Gioia Taura, whence the wood was shipped to Naples. At the time, other shipyards were opened closer to home (but nearer some trees)—the yard at Castellammare, for example. Nevertheless, ships were in such demand that the Aragonese (rulers of Naples from 1442 to 1500) actually bought ships from elsewhere in the Mediterranean.
The entire arsenal succumbed to the "scorched earth" policy of the Aragonese when they burned the yards in 1498 rather than surrender the facility to the invading armies of Charles VIII of France during his short-lived attempt to take over southern Italy.
Spanish rebuilding of the arsenal was substantial and involved moving the facility slightly to the west, closer to the shelter of Mt. Echia, the cliff overlooking Santa Lucia; the facility then occupied Molosiglio, now a port for pleasure craft and the site of a large park built on landfill. Interestingly, this new arsenal fell on hard times because of its inability to handle and outfit the larger and newer ocean-going vessels occasioned by the imperial expansion of Spain at the time. Naples, also, was not in the strategic position of ports in Spain. Commercial sea routes were also negatively affected by the reopening of a number of old Roman land routes on the peninsula. In the 1600s, nevertheless, the Naples arsenal—some of it designed by Domenico Fontana, one of the great names in Italian Baroque architecture—had become a small city unto itself, with housing for the considerable number of workers, shops, a chapel, even its own local courthouse.
Expansion of the arsenal under the Bourbons (who took over Naples in the 1730s) was impressive and directed by John Acton (1736-1811), commander of the Neapolitan navy and very concerned with beefing up the military might of the kingdom such that it would prove a worthy ally of Britain in the struggle against Republican France and then Bonaparte. By that time, however, the facility at the port was already too small to handle shipbuilding needs for a major sea-going navy. Indeed, shortly thereafter, in 1818, the first ocean-going steamship in Italy (the San Ferdinando, rechristened Ferdinando I) was built in Naples —not at the arsenal, however, but rather a mile away in the Stanislao Filosa shipyards at the eastern end of the port. Molo S. Vincenzo, the area directly in front of the naval college, was built up between 1826-51 to be a new military port, but as a shipyard, it was already too small. Also, it couldn't handle the transition from sailing ships to steamships. The yards of Castellammare could and did, relying on the large foundry at Pietrarsa (now a train museum), which made all the boilers for locomotives and steamships in the Kingdom of Naples.
At the time (1860) of the annexation of the Kingdom of Naples to the rest of Italy, the arsenal still employed 1,600 workers. The port, itself, had a state–of–the art 75–meter dry-dock, and the general situation of the Neapolitan merchant marine and navy was not all that bad—4/5 (!) of all tonnage in Italy was in the hands of the Kingdom of Naples, and the Castellammare shipyard was the largest in Italy, employing 1,800 workers.
A number of things contributed to ultimate demise of the arsenal. One, unification, itself, reduced the importance of the yards; that is, naval needs of a united Italy could be handled elsewhere (though the large facility at Castellammare continued to be important, and still is). Two, in 1873, with the fear of a war with France, a new military port/arsenal was built at Taranto at the extreme southern tip of Italy; the Naples arsenal was not felt to be defendable. Three, the rebuilding of the Santa Lucia area of the city; that is, the depositing of landfill and the construction of new blocks of buildings where water used to be, as part of the risanamento, effectively ended the existence of the Naples arsenal.
note: I gratefully acknowledge my reliance for much of this information on L'Arsenale della Marina e l'Economia Del Regno di Napoli (secc. XV-XIX) [The Naval Arsenal and the Economy of the Kingdom of Naples (15th-19th cent.)] by Nicola Ostuni.