At first glance the citadel of Dunkirk in 1713 appeared to be a disorganised arrangement of bastions and half-bastions. However the final form of the citadel was in fact a product of its history, which began during the English occupation in 1660. The English engineer Bernard de Gomme was keen to exploit the area of ground on the west side of the harbour entrance. This area was difficult for a besieger to attack without first taking the town, and it was also important for protecting the harbour entrance.
The Spanish had built several small redoubts there, but the English decided to build a major work, calling it Fort Lyon (based on an earlier Spanish name for the area; Fort Leon). De Gomme's Fort Lyon was essentially an extension of the bastioned trace on the west side of the town, with two large bastions. The left flank of the left-hand bastion extended back to the harbour in order to create a self-contained fort. The English engineers made two good bastions, revetted in stone with false brays in the flanks. However, the main weakness of Fort Lyon was that it was partially flanked by one of the town's bastions, which rendered it useless if the enemy captured the town.
When the French took possession of Dunkirk in 1662 they carried on the work on the citadel, adding two small bastions facing over the harbour mouth and two bastions in the rear. However, the most serious defect, that the left bastion was flanked by a bastion outside the citadel, was not rectified. In the 1670s the French engineer Vauban remodelled the citadel. It was his solution to the poorly-flanked original design that led to the final form of the citadel. Rather than demolish the existing two English bastions, Vauban converted them into half-bastions. In front of them he added a hornwork, thus lengthening the overly short southern front of the citadel and simultaneously adding another layer to the northern defences.
The English bastions and the curtain joining them were left in place as an inner line of defence. A large cavalier was built on top, with additional cavaliers on each bastion. This resulted in a triple layer of firepower facing south, west and north from the citadel. It is testament to Vauban's skill as an engineer that he was able to turn the poorly flanked English bastions into the citadel's greatest strength.
The cavaliers, additional ramparts built on top of the main rampart to add more height, were one of the most striking features of the citadel. To the west, they gave the garrison a height advantage against an attacker using the sand dunes for cover. To the east they increased the firepower and visibility over the harbour and the town. The additional height would also have given the garrison a better view of what was happening out to sea.
The citadel had four demi-lunes; one facing towards the naval arsenal, one facing out to the west, one on the north side and one facing the harbour mouth. There were also small coastal batteries on either side of the harbour mouth to deter any hostile ship that had somehow managed to fight its way through the network of sea forts.
Inside the citadel there were barrack buildings, storehouses, powder magazines and the governor's quarters. The garrison of Dunkirk was large, but the space left between the English bastions and the harbour walls was small, so the French had little space to work with. As a result, the arrangement of the interior buildings was rather cramped.
The entrance to the citadel was via a wooden bridge over the ditch and gate in the wall facing the harbour. There was no gate in either the citadel or the naval arsenal's fortifications that faced west, towards France. Vauban considered this to be a serious deficiency. In the event of an attack a gate on this side of the fortress would make it much easier to slip reinforcements into the town before the besiegers cut it off. However despite Vauban's recommendations no gate was built and the citadel remained accessible only from the town.
Although it was never tested in war, the citadel of Dunkirk was undoubtedly a strong work and one of Vauban's finest achievements. It would have been extremely difficult to capture, even for an enemy who was in possession of the town and the naval arsenal.