In the early 17th century the northern edge of Flanders formed the frontier between Spain and the United Provinces in the 80 Years War. In 1604 Prince Maurice captured the important fortresses of Sluis and Aardenburg, which lay in a strategic location to the south of the Scheldt estuary.
It was obvious to all concerned that the Spanish would mount a counterattack and attempt to recover their lost territory, all the more so since the recent surrender of Ostend had made a large number of Spanish troops available for new operations.
Consequently, Prince Maurice undertook to strengthen Sluis and to construct a number of small forts in the area. One of these was an entrenched camp at the small village of Cassandria, to the north of Sluis and on the opposite bank of the Zwin channel (see map above left).
It is likely that the site had been occupied by the Dutch during the siege some months earlier, but now it was to become a more permanent fortification. The western boundary ran along the east bank of the Zwin and the landward sides were protected by simple earthwork banks.
The camp at Cassandria became known as Retranchement, a French word meaning entrenchment. The fortification at Retranchement allowed the Dutch to control the Zwin, which was the shipping route that was needed to supply Sluis. The first of many Spanish attempts to recapture Sluis came in 1606, justifying the construction of fortifications.
At some point two square bastioned'forts were constructed on the western side of the entrenchment; Fort Orange to the north and Fort Nassau to the south. Both forts and the ramparts around Retranchement were protected by a flooded ditch.
A Twelve Years Truce was signed between Spain and the United Provinces in 1609 but in 1622 the two nations were at war again. The Spanish built the fortified Lines of Vuile Vaart in an attempt to tighten the noose around Sluis. These lines consisted of a series of redoubts'and the northern end was anchored by Fort Isabella, a square fort situated directly opposite Retranchement.
It was probably in response to the construction of this fort that Retranchement was strengthened by Fort Orange and Fort Nassau. Although there were no more attempts on Sluis after the 1620s, the frontier was still active, and Retranchement was strengthened again in the 1640s.
This time it was the eastern side that was to be improved; the earthworks there were weak and irregular in form. It was decided to construct a trace of three bastions, which offered greater protection and enabled the entrenchement to face a serious land-based attack from the east.
The western side was left as it was, a single straight rampart with a kink halfway along it, flanked by the two forts at either end. This side of the entrenchment faced the Zwin channel, so it was unlikely to face anything more than a surprise attack.
In 1648 the Treaty of Münster, part of the Peace of Westphalia, brought an end to the war. It stipulated that the fortifications at Retranchement be demolished, but for some reason this article was never carried out and the fortifications remained intact.
However, what the treaty had failed to do, nature was to prove capable of doing. In 1682 a huge storm washed away Fort Orange, which lay at the northern end of the entrenchment. By this time the frontier defences here were not so important, so this was not critical.
When repairs were carried out in 1684, the gap left by the Fort Orange was simply closed up with a rampart and the fort was not rebuilt. The fortifications of Retranchement were declassified in the 19th century. It is interesting that this type of entrenched camp would become common in the late 18th century, an example being Belfort.
Apart from Fort Orange, which was destroyed by a storm in the 17th century, the fortifications of Retranchement have survived relatively intact. The Zwin is now completely silted up and is occupied by fields, making it hard to imagine seagoing ships passing the fortifications.
There is a footpath (called the Wallenroute) leading along the top of the rampart and around the outside of Fort Nassau (the inside cannot be visited).
The ramparts are rather overgrown, particularly in the north, but this means that the earthworks have not suffered from erosion since the roots of the vegetation give them strength. Retranchement is within walking (or better, cycling) distance of Sluis.
The remains of Fort Isabella stand in a field next to the road about 1½ miles (2.4km) to the east, but there is not much to see.