Gravelines is a small town on the river Aa near the sea. The first artillery fortifications were built there by the Spanish under Charles V'between 1528 and 1536, when an early artillery type brick castle was built to guard the western entrance of the town.
In 1556, a hexagonal trace of six arrow-headed bastions'was designed for the town by the Italian engineers Olgiati, Thebaldi and Saluberto. Since the river Aa took a circuitous route to the sea, a more direct channel was dug in 1637 to improve shipping access to the town. A small fort called Fort St-Philippe was constructed at the mouth of this channel in 1640, after an attack on this area by French troops in 1638. It consisted of a square bastioned'fort on the east bank and a hornwork'on the west bank.
The town's defences were also strengthened in the 1640s by the construction of some demi-lunes'and the covered way. A hornwork on the western side of the town was also added at this time, facing towards France.
Even as these improvements were being made, French armies were pushing their way northwards into Spanish territory and in 1644 a force under Gaston d'Orleans laid siege to Gravelines and captured it.
However, the town did not remain in French hands for long and was retaken by the Spanish in 1652. In 1658 the French returned under Louis XIV'and captured Gravelines once more. The town was annexed to France permanently by the Treaty of the Pyrenees'in 1659.
Vauban'was to make Gravelines the northern anchor of the second line of his Pré Carré. In 1699, he designed an extensive system of sluices to allow shipping from the sea and also to allow the approaches to the town to be flooded to prevent attackers from using parallels'to dig in their batteries.
In 1706, a year before his death, Vauban was appointed governor of Gravelines and given command of the coastal sector of fortifications in the north of France, which were threatened by the allied army under the Duke of Marlborough'.
Gravelines was one of the towns that served as a base for pirates and privateers hunting Dutch and British shipping in the English Channel, which was one reason why Fort Saint-Philippe was strategically important, allowing shipping to pass up the channel to the town in safety.
Vauban, in a letter to Louvois'(Louis XIV's foreign minister), opined that the corsairs of Gravelines, if they had financial help from the French government, could do more damage to the Netherlands than all the French land armies put together.
One of the main reasons for Marlborough's advance on Ostend and the coastal sector in 1706 was that privateers operating out of French ports such as Gravelines and Dunkirk were damaging Britain's trade. He argued that they could be stopped by depriving them of these French-held havens.
An interesting feature of Gravelines is the castle, which is built into one of the bastions. Using the western bastion as an outer defence, there are walls on the townward side as well. This unique citadel'has its gate at the apex of the two straight walls on the townward side.
The inner defences of this citadel however are weaker than the outer fortifications, being remnants of the pre-bastion castle built in the 1530s. Walls run straight from the inner sides of the main walls to meet behind the bastion, where there is a gate between two large horse shoe shaped towers, which were not perfectly flanked and therefore vulnerable.
The inner walls of the citadel are flanked by these towers and by demi-bastions'built into the main wall. There was a shallow dry ditch in front of the inner walls of the citadel, which caused the outer wall to have a lower section where the ditch met it.
Gravelines lies between Calais and Dunkerque, easily accessable by road but there is no station. The fortifications are almost entirely intact, save the western outworks and part of the eastern entrance. Walking access to the fortifications is good for most of the fortifications.
If you're feeling a little more adventurous, it is possible to hire small boats to sail round the flooded ditch in the summer. The tourist office in the town has some useful information on the fortifications including a map, so its well worth heading there first.
A visit to Gravelines fits in well with other Vauban fortresses in the region; Bergues (south of Dunkerque) and Calais. There is a museum in the citadel that contains the relief map shown above, and also a digital 'virtual copy' of the map on computer.