Naval Fortifications is quite a wide topic, spanning centuries. It started in fact right at the age of fortifications, when the defenders saw a cliff or some point on a coast was easy to defend, to create a fort. The sea is the best moat available, more so, if the cliff is associated to it. The first naval forts however went back to the antiquity. The Romans in particular dotted islands of castra
(forts) after Pompeio Magno (Pompey the Great) "cleaned up" the Mediterranean from pirates in xxx BC, in particular the infested aegean sea and Asia minor coast down to the syrian coast. Many were converted in medieval forts. The medieval era is well known for its castles, like those William I of England built to rule his newly conquered island after 1066, but it's really when the age of gunpowder commenced before the Renaissance, that gunnery started to appear on coastal forts, seen as a way to sink or repel ships. The idea was to use these "unsinkable man-o-war" to defend restricted access to harbours and coastal cities, often created in naturally protected bays. A couple of well-placed naval fortifications indeed could interdict an entire area to whole fleets, the gateway to a country.
In the XVIIth Century when fortifications became a science (and the art of building them and take them), naval fortifications multiplied as well, at an age of global empires and fleets in being. When better guns in the XIXth Century (Industrial era) became superior to any fortifications, naval forts were refined and never completely lost their potential. The American Secession war in 1860 started by the assault of Fort Sumter, and the Crimean war was mostly a naval assault on fortifications (Russians), as well as in WWI, the same allies assaulted Turkish Ottoman forts... to be defeated by mines. In the interwar, fortifications were still relevant and still built or modernized, just as the Maginot line or Eben Emael and many other modern forts were constructed at great cost and ingenuity. At sea, the USN invested much in naval fortifications and perhaps reached the pinnacle of the genre with its "concrete battleships" of the Philippines. Fortifications stayed active even in the age of the atom, during the cold war, but today, they are definitely seen as a relic of a bygone era, after being in service for a thousand years in some places.
Archimedes directing the siege of Syracuse
Before the age of gunpowder, forts were just coastal garrison cities, but could offer some protection against an aggressors with their slings, bow and arrow, and later and ballistae and catapults when they appeared in the Vth Century BC. The Greek and Phoenicians built commercial empires and harbors all around the Mediterranean, placed on the best covered bays, often using narrow entrances. The latter were the best places to built at least an observation tower, to warn the defenders of the city state of an approaching fleet, such as Troy when the unified Greeks arrived on their shores. Famous city-states of the classical era drove their wealth and power from trade, and affirmed their mastery not only by their fleets, but their high walls.
When Alexander the Great tried to conquer the Persians, he tried to eliminate the all-powerful Persian fleet by taking coastal cities by land, one by one. The siege of Tyre ()
showed how far a coastal fortified city of that era could withstand and assault. Later, Rhodes was also besieged and held off all attackers. The diadochi era saw a flurry of siege and fortifications building, with sometimes continuity between a coastal city and harbour, like the long walls running from ancient Athens and the Piraeus, destroyed by treaty at the end of the Peloponnesian wars, won by Sparta backed by Persia. One of these prominent warlord was even called Demetrios "poliorcete", the "city taker", and the size and capabilities of siege engineering reached an all-out summit. The Romans would later leanr a great deal about these technologies and improve them.
The "hand-claw" allegedly designed by Syracuse to defeat Roman ships
In between the second and third Punic war, naval fortifications reached perhaps an all-out peak with the fortified harbour of Carthage: The famous round harbour which protected the fleet, virtually impossible to take for any assaulting force. A wonder of construction of the antiquity, it was completely embedded on the city walls itself, the only access by sea was through a narrow passage that could be blocked by a chain, fortifications with archers and ballistae on both sides, and then the civilian port, and the military port inside a circular precinct. During the same era, the siege of Syracuse (214–212 bce) showed how far could be pushed the defense of a besieged coastal city, led by Archimedes himself. Although a part of the Roman tale is dubious at best, we can wonder how Archimedes contraptions were effective. Fact is, they were never repeated anywhere else.
The military port of Carthage in 147 BC.
Medieval Sea fort:
The Roche Goyon Fort in Britanny (1340). Its location made it impregnable and it played an important role during the hundred years war.
Preventing sea invasions involved two types of forts: Primitive "castra" partly in wood close to the coast or back to a cliff corner, in order to reduce the land surface to defend, and river forts, that can defend an inland city. Such forts were built upstream near Paris during the Viking invasions and contributed to repelling these. But another way to defend the land was to have a major fort deep inland and coastal outposts with a string of alarm fires along the way. Once an invasion was detected, the alarm fire was the quickest way to mobilize the army. Well before the Vikings, the Aegean sea "sea peoples" became a nuisance and coastal defences probably erected by civilizations neighbouring the Mediterranean.
The main problem of riverine or coastal forts was with bow and arrows only, it was difficult to interdict a sea invasion of any sort. Ballistae and Catapults were probably required to get the extra reach to do some harm to the approaching ships. The idea of using a chain to block access in a natural bottleneck is another fairly old idea, as well as using a boat barrage across the river. At sea however, there was little ways to block the access of a fleet. The Fort La Latte (1340) was an interesting fort of the late medieval period, in stone and built over granite in a spectacular location, more usable to see the surroundings and defend itself than to prevent a fleet to pass by. Other examples includes the Crusaders's 1228 Sidon Sea Castle in Lebanon, Mt St. Michel in Normandy, Bodrum castle in Turkey, Mackenzie castle in Scotland and others.
XVII-XIXth Cent. Naval Fortifications:
A Vauban fort: The typical close quarter fortification built from the XVII to the early XIXth centuries.
Around 1327 the first cannons appeared in Western Europe, and in time, they not only revolutionized naval warfare, but also land warfare, and naturally they became the main reason why naval fortification mattered again, and this until WW2. For once, cannons could reach ships at a distance no other medieval weapon was capable of, including a trebuchet. There was a newly-found interest to fortify specific points protecting a bay, an island, a natural passage towards a port or cove. This came in addition to another benefit: Contrary to a ship, a castle cannot sink. Al long as technology allowed forts artillery to stay relevant against ship-borne artillery, this was an even match.
The spectacular Fort Louvois (1690), protecting the Charentes inlet, main way to Bordeaux. It was opposite to the Oleron island citadel for a crossfire, and one of the many first build under Louis XIV. Louvois was replaced by Vauban which dotted the coast with more citadels and sea forts, protecting the entrance to the seine river.
A late 1880s naval fortification: Heavy guns are using an hydraulic recoil system allowing them to be quickly back in position after loading, the latter made just as the barrel ended its recoil course. It allowed to quick fire heavy guns, but the installation was too extensive to be installed in a cramped turret or even barbette, so better suited for naval fortifications. The British had two such guns defending Gibraltar in case of an attack by the Italians, which in 1882 had the largest naval guns ever on a ship, the barbette ironclads Italia and Lepanto.
Naval fortifications played their role at multiple occasions, especially when the Ottoman Navy was head and shoulders in one of the most fantastic and decisive sieges in history: The Fall of Constantinople. Naval fortifications played a huge part in this, especially as the city was basically split in two, controlling access to the black sea. For once, the walls were nearly impregnable, and the Byzantine created a chain barrage at the entry of the golden horn. Further below, on reclaimed Turkish-held shores, the Ottomans built two forts, one on either side, with powerful artillery to impose a blockade. To circumvent the problem of the barrage, ships were hailed outside the walls of Pera, on the Asiatic side, across wooden ways, and to the gold horn, and a boat bridge was bridge further north. After the fall of the city, largely due to the land siege, the Ottomans learned their lessons and fortified not only the golden horn but also the Gelibolu (Gallipoli) peninsula. Old forts were rebuilt and modernized with German Krupp guns before 1914 (see later).
The siege of La Rochelle, in 1627. Painting by Henri Motte in 1881. The protestant stronghold, bolstered by British forces, held the French Catholics led by Cardinal Richelieu (which plays a large part in the funding and organization of the French Navy), lasted for about a year, from 10 september 1627 to 28 october 1628. To prevent supplies by sea, Richelieu started on November, 30 the construction by 4,000 workers of a dike 1,500 meters long and 20 meters high with foundations on sunken, backfilled ships and guns positons pointed towards the sea were to counter any reinforcements. Meanwhile a French fleet led by admiral Marino Torre blocked the harbour entrance. That was basically a repeat of the siege of Alesia by Caesar, but at sea. Indeed, the british soon sent reinforcements, 80 ships under command of George Villiers, 1st Lord of Buckingham, which took the nearby Island of Re, and later a second force by William Feilding, 1st Earl of Denbigh, in April 1628, arriving in May with 60 ships and 6 armored barges. Both failed and the last was sent in August by admiral Robert Bertie, 1st count of Lindsey with 29 warships and et 31 merchant vessels. He battled against the Dike and withdrawn in September. The city capitulated one month later. Richelieu showned a fortfied, gun-armed dike could hold its own against fleet assaults. The lessons were well laerned.
During the The Russo-Swedish War of 1741–1743, naval fortifications also played their part. Vyborg and St Petserburg were not only strategic harbors and arsenals, their approaches were also riddled with Vauban-style fortifications. For the Russians, the waterways in the Gulf of Finland became of strategic importance and small islands constituted unsinkable battleships, trigerring the construction of specialized artillery forts during the XIXth century. The Fort Alexander I, defending the access to St Petersrburg was one of these Kronstadt string of fortifications, quite important at the time, built 1838-45. It was active during the Crimean war
, which took place both in the baltic and black sea (sevastopol).
In 1901 was created the Kustartilleriet, born from traditionsgoing back to the old coastal fortresses in use around Sweden since the 15th century. The command structure subordonated until then to the fortress artillery department. This became an independent branch, starting with the Vaxholm Artillery Corps in 1889. Both coastal defence fortresses and city fortresses were formerly under command of Artillery branch. But with the arrival of the fixed mine defence units the question of an independent branch returned to manage both mines, torpedo and artillery in coastal fortifications, ending with the constitution of the Coastal Artillery in 1902, the fusion of the Vaxholm Artillery Corps, Karlskrona Artillery Corps and fixed mine defence units.
Typical harbour the the early XXth century. Yards, cranes, warfs, rail, were all interconnected, large hangars were used for smaller ships construction, large storehouses for provisional bulk storage, large fuel tanks, oil tanks, captain's shack and administrative buildings, were all part of the landscape, but no fortifications. Since these vulnerable installations were located inside a cove or bay, the field of fire would have been less than optimal, and if the ship reached that point, that was over already. But this was only due to the development of long range artillery during the late industrial era. At the start of the XIXth Century, cannonballs and smoothbore muzzle-loaded cannons were the norm and both accuracy and range were limited to 5 kilometres at best. This made harbour fortifications useful, and most of the old coastal cities of Europe were both walls, and fortified on the seaside as well. Ships went into anchorage literally "under the guns" of the harbour in that case. In the Caribbean, at the height of the piracy in the XVIth Century, pirate fleets went duelling with such fortifications, or pound the city itself.
Here a late XXth century Lighthouse
The Hanko Lighthouse, surrounded by fortifications was a focal point in the baltic, between the Finns and Soviets, for the control of the Gulf of Finland during WW2.
Naval Fortifications of WW1
A Typical fortified bunker and associated AA. This was a widespread type or naval fortifications, especially in Scandinavia.