Dunkirk
By the mid-17th century the channel port of Dunkirk had a reputation for being a haven for privateers who were a menace to Dutch and English shipping. However, when Dunkirk was acquired by France in 1662, after a brief period of English rule, the harbour entrance was becoming silted and large ships could not enter. French king Louis XIV decided to transform the town into an important naval harbour and fortress, spending vast sums of money there throughout the rest of the 17th century. As a fortress, Dunkirk formed the northern anchor of the first line of Vauban's Pré Carré, a system of fortresses protecting northern France from attack. As a naval harbour, Dunkirk provided a valuable base for ships operating in the English Channel, an area where France had few good harbours.
Vauban, the king's military engineer, fortified the town with 10 large bastions, based on the existing Spanish works, and built a citadel next to the harbour entrance. In order to achieve the king's naval aspirations for Dunkirk a large basin was excavated in 1670s that could hold 30 warships. To prevent the basin from running dry at low tide, a huge double lock was built at its entrance. To enable large vessels to come into the harbour a channel was dug across the Banc Schurken, a sandbank that lay just off Dunkirk. This channel was secured by two jetties, which ran about a kilometre from the town to the deep water. When this work was complete in 1678, large vessels could use the harbour and France had a powerful naval base from which to strike at English and Dutch shipping.
The French recognised that the jetties were the lifeline of Dunkirk's harbour. In wartime an enemy naval force would only have to destroy or damage the jetties and the channel would quickly silt up. There were also concerns that an enemy would try to bombard the town from the sea. During the 1680s the French navy bombarded Algiers, Tripoli and Genoa using a new type of ship, a "bomb vessel" (two of which were built at Dunkirk). It was obvious to the French that Dunkirk was at risk from a similar bombardment. Because the channel was so long, it was not possible for the guns in the town's fortifications to cover the length of the jetties or keep enemy bomb vessels out of range. To provide protection for the town and the jetties, a number of forts were built in the sea in the 1680s.
At first glance, these forts appear to be scattered at random across the area between the town and the low tide mark, but in fact each fort was built to cover a specific approach. Between them they protected the jetties and the harbour from any conceivable seaborne threat. Each of the six sea forts is described below, with the help of computer-generated 3D models that are based on original plans and drawings.
Château d'Espérance
The Château d'Espérance was a large wooden fort built at the head of the west jetty. It took the form of a wide curved battery facing out to sea. Behind the battery was a long building with accommodation for the garrison, stores and a powder magazine. There were two demi-bastions to protect the rear of the fort, although no cannon faced the rear. The whole fort stood on a forest of stilts, which were driven into a substantial piling base to secure the foundation in the shifting sand beneath. There was a line of vertical stakes on the seaward side to stop fireships from running into the fort. A wooden bridge linked the fort to the jetty itself.
The Château d'Espérance covered the deep water between the low tide mark and a sandbank that lay off the coast, called the Brack. The Brack was just within range, so a ship could not sail past the end of the jetties without coming under the fort's guns.
During the Anglo-Dutch attempts to bombard Dunkirk in the 1690s, the Château d'Espérance bore the brunt of the attacks. In 1694 the Allies sounded the approaches and realised that it would not be possible to bombard Dunkirk without destroying the forts at the jetty heads first. Fireships were sent against it, but they were all sent off course by the waves or by French longboats towing them aground. During the 1695 attack Jean Bart, the famous Dunkirk privateer, was put in command of this fort.
Château Vert
The Château Vert stood opposite the Château d'Espérance at the head of the east jetty. Like its neighbour, the Château Vert was a wooden fort supported by stilts on a base of pilings. It was longer and narrower than the Château d'Espérance and most of its guns did not face out to sea, but to the east. The reason for this was to cover the area to the east of the jetties, from which ships could bombard the town. As with the Château d'Espérance, the rear of the fort was protected by two demi-bastions of loopholed walls, with a bridge leading to the jetty.
The Anglo-Dutch attacks on Dunkirk in the 1690s all came from the west, so only the north-facing guns of the Château Vert were used. The attackers sent fireships against both the forts at the jetty heads, but the heavy fire coming from both forts forced their crews to set them on fire while they were still a long way off, so none of them reached the forts. During the 1695 bombardment the Château Vert was hit by a shell from a bomb vessel, which did no damage apart from making a hole in the fort.
Grand Risban
The Grand Risban was a massive stone fort built between the town and the low tide mark. It was the largest of the Dunkirk coastal forts and an impressive feat of engineering. The fort took the form of a triangle with curved faces, reflecting the three specific reasons for its construction; one side faced north to supplement the jetty head forts, one side faced south to cover the sand dunes to the north of the citadel and the third side faced east to cover the foreshore east of the jetties.
The fort consisted of a single battery on the top, mounting 45 guns (15 per side) and there was a garrison of 100 men. In the centre was a sunken courtyard surrounded by barrack buildings and storerooms three storeys high. This arrangement protected the buildings from enemy fire but still allowed for windows facing into the courtyard. The commander's residence was a larger building in the centre of the east side. There were powder magazines in the vaulted cellars beneath the barracks. Three staircases in the corners enabled the garrison to access the battery as quickly as possible. The fort was linked to the west jetty via a wooden bridge that led to a gate in the wall. Through the gate was a tunnel that led directly into the courtyard.
The Grand Risban was an impressive engineering acheivement, but in reality it was poorly situated to cover the three areas that it was built to protect. To the north, the jetty head forts were very effective in protecting the entrance to the channel and the Allies never managed to destroy them, so the Risban was of little value there. To the south, the Risban was too far away from the town, so its guns could not effectively cover the dunes there. The Fort de Revers was built to carry out this role. 
To the east, the area was too large to be defended by the guns of the Risban from the opposite side of the channel, so the Château Gaillard and later Fort Blanc were built to provide additional protection. It seems that the fort's location was a compromise between these three functions, so that it was unable to carry out any of them effectively. Nevertheless, the Risban was a formidable work that was feared by the Allies. They knew that if they were ever to succeed in destroying the jetty head forts they would meet a much more formidable obstacle in the form of the Grand Risban.
Château Gaillard
The Château Gaillard, situated about halfway along the east jetty, was the smallest of Dunkirk's coastal forts. Mounting just 8 guns, it was a rectangular wooden platform on stilts. There was a small barrack building and a bridge linking the fort to the jetty. The Château Gaillard was built on a platform that was originally used during the construction of the jetties. Its purpose was to cover the area to the east of the jetties and protect the east jetty from an attack. 
This area was dry at low tide, so the attack that was foreseen here could have been made by small vessels at high tide or by troops at low tide. The fort's small guns could even have offered some support to the land defences of the northern side of the town. Since Dunkirk was never attacked by land and the naval attacks all concentrated on the jetty heads, it is unlikely that the Château Gaillard ever fired a shot in anger.
Fort de Revers
The fortifications of the town and citadel of Dunkirk had a weakness where they met the coastline. At this point there were large sand dunes on the beach. An attacking force could advance along the coast, concealed from the defences by the sand dunes, and come very close to the town. The French themselves had used this tactic against the Spanish garrison of Dunkirk in 1646 and 1658, so they were well aware of this weakness. It was originally thought that the Grand Risban would be able to provide fire coming from the sea to cover the north side of the sand dunes, but in the end it was built too far out to sea for its guns to be effective in this role. So another sea fort was built entirely for the purpose of covering the dunes and protecting the landward defences. This fort was called the Fort de Revers (the Reverse Fort) because although it was built in the sea, all its guns pointed back towards the town.
The fort took the form of a triangle, which was dictated by its function. There were two straight sides that mounted guns, facing south-east and south-west. To the south-west, the fort covered the dunes next to the citadel and to the south-east the fort covered the dunes to the north of a sector of defences known as the Tete de Nieuport, which was where the French had attacked from in 1646 and 1658. On the third side, facing towards the sea, there was a loopholed wall for musket defence, with two half-bastions.
The Fort de Revers was made of stone, like the Grand Risban, and contained accommodation and stores for the garrison. The fort is recorded as mounting 24 guns, presumably half to flank the citadel and half to flank the Tete de Nieuport. However, most plans only show 16 embrasures, so where the extra guns were placed is a mystery. Although the north-east corner of the fort was very close to the jetty, the entrance bridge actually ran parallel to the fort's east face, turning sharply to reach the gate, which was situated at the south corner. It is possible that the gate was situated here to maintain the perfect symmetry of the fort. Another possibility is that having the bridge running directly below the fort's wall offered additional protection.
Fort Blanc
In 1701 France was threatened with a new outbreak of fighting against the naval powers (England and the Netherlands). This gave rise to fresh fears for the safety of France's coastal towns and harbours. Fort Blanc was built as a result of renewed fears of an Allied naval bombardment of Dunkirk. It addressed a perceived weakness in the defences: the area to the east of the jetties, where a fleet of bomb vessels could theoretically come close enough to bombard the town.
Fort Blanc, which was named after the naval intendant le Blanc, was the only one of the sea forts that was not accessed by a jetty. It was dry for several hours at low tide, so it would have been possible to walk to it, or to reach it by boat at high tide.
The fort was built of stone, with a barrack building on top and vaulted rooms underneath. The entrance was via a stairway that led from the gateway up to the top of the fort, where the guns were located. The main battery of the fort was a wide curve that faced north, out to sea. To the rear there was the gate, covered by loopholes for musketry defence in the outer wall of the vaulted lower storey. The guns at the ends of the main battery could also be turned to face southwards, if an enemy force came between the fort and the shore.
The Fate of the Sea Forts
The sea forts were completed in the 1680s, with the exception of Fort Blanc, which was built in 1701. They successfully defended Dunkirk from several Anglo-Dutch naval attacks in the 1690s, so that not a single shell was able to reach the citadel or the town. However in 1713 the Treaty of Utrecht was signed, bringing an end to the war between France and Great Britain. According to this treaty the fortifications and port facilities of Dunkirk had to be completely demolished. This meant an end to the jetties, the sea forts and the land fortifications. In the mid-18th century new sea forts were built, partially utilising the ruins of the earlier forts, but these were also fairly short-lived. Today nothing remains of Vauban's grand system of forts built in the sea at Dunkirk. The Risban lighthouse is built on top of the ruins of the Grand Risban, but there are no traces of the fort itself.