After the creation of the Zwin channel during a violent storm in 1134, a settlement known as Lamminsvliet grew up at the place where it met the Scheldt estuary. It was first fortified towards the end of the 14th century, at which time the town began to be known by its current name, Sluis.
These fortifications included a flooded ditch'running around the town, three gates and a large castle on the north side. Sluis prospered from the trade enjoyed by the Flemish city of Bruges in this time. All shipping to and from Bruges had pass through the Zwin channel and hence past Sluis.
In the middle ages, seagoing ships unloaded their goods farther up the Zwin at Damme, where they were loaded onto smaller vessels and transported to Bruges. After the 13th century however, the channel began to silt up and became unnavigable for larger ships, so Sluis took over the role of seaport for Bruges.
At the beginning of the 80 Years War, Sluis had some rudimentary artillery defences, notably two arrow-headed bastions'guarding the entrance to the inner harbour, which were constructed after a failed attack by the sea beggars'in 1572.
Sluis joined the revolt in 1578 and in 1585 it was garrisoned by the Earl of Leicester's English troops (England was helping the rebels), but the town still fell to the Spanish in 1587 after a long siege.
In 1604 Prince Maurice, in an attempt to relieve the siege of Ostend, landed in Flanders with a large army and captured Sluis. The town was of strategic value to the States because it protected Zealand to the north and provided a bridgehead from which attacks could be made in the future.
In addition to this, being in possession of Sluis helped them to secure the Scheldt, which was the approach to Antwerp and gave them control of the Zwin, which was the route to Bruges. The fortifications of Sluis were reinforced as quickly as possible to guard against a possible counterattack from the south.
Over the next months Sluis was comprehensively fortified according to the latest military engineering theory, Old Dutch School of Fortification. The new fortifications consisted of a well-proportioned bastioned'trace protecting the landward side of the town.
There were 6 bastions and 2 demi-bastions'where the fortifications met the harbour. A continuous false bray'ran at the foot of the ramparts. The medieval castle at the northern end of the fortifications was retained. Although its defences were obselete, its situation at the angle between the harbour and the Zwin meant that it was not in a position to face an attack and did not constitute a weakness to the fortress.
The west side of the town, which faces the old harbour, was fortified with small bastions. There were a number of works on the western side of the harbour, protecting the landward approach, including a hornwork'and some small bastions, fronted with flooded ditches.
All these works on the far side of the harbour were open to the rear, so that they could not be used against the town if they were captured by an attacker. All the new fortifications were earthworks (as was standard with the Old Dutch School) so they could be constructed relatively quickly using unskilled labour.
This was just as well, since a Spanish force attacked Sluis in 1606, almost taking the Oostpoort (East Gate) in a surprise-attack at night. The Spanish returned in 1621 and again 1622 but each time they failed to retake the town. Sluis was firmly in the hands of the States.
To counter the Dutch presence at Sluis, the Spanish fortified their own positions by building a number of forts and fortified lines. In 1622 Fort St Fredrick was built less than 3 miles (4.8km) from Sluis and a line of redoubts'called the Lines of Vuile Vaart linked this fort to another, Fort Isabella to the north of Sluis.
This line was replaced in 1632 by a new set of lines, the Lines of Fontaine-Cantelmo, consisting of an earthen rampart with redans. These took a different route, keeping farther from the town of Sluis - perhaps the earlier lines had been a little too close for comfort.
In 1672 during the Dutch Wars'Sluis and the nearby fortress of Aardenburg resisted French attacks. From 1699-1702 the fortifications were strengthened by the Dutch engineer Menno van Coehoorn, who built demi-lunes'and counterguards.
These modifications increased the depth of the defences by placing the covered way'at a greater distance from the main wall and placing more obstacles in the ditch that an attacker would have to capture. The castle was left in place, but its towers were lowered.
During the War of the Spanish Succession'Sluis was captured by the French, who occupied the Spanish Netherlands and went to war with the Dutch, Austrians and English.
The French (or rather, an unknown engineer) drew up plans for strengthening Sluis by building recessed flankers in the bastions (effectively turning them into arrow-headed bastions) and with a square citadel'around the castle. However, the French did not occupy Sluis for long and no work was carried out.
Sluis was captured by the French troops in 1747 and again in 1794. The second time Sluis surrendered to the French with hardly a shot fired, although not before the castle had been damaged. In 1816 the fortifications were declassified and the remains of the castle were demolished in 1820.
The fortificatons of Sluis have, for the most part, been preserved in excellent condition. The whole of the landward bastioned trace has survived, although most of the outworks are gone, with the exception of the Holland, Zeeland and Friesland demi-lunes.
The sections that have been covered with trees for a long time are the best preserved, since the roots bind the earth and protect the ramparts against erosion. The old harbour silted up long ago and is now farmland. The fortifications on the far side still exist, but they are somewhat decayed.
The gates are well preserved and have been restored, although the bridge in front of the Oostpoort (East Gate) has not been fully rebuilt so as to reach the demi-lune. There is footpath leading along the top of the rampart and a cycle track that follows the line of the covered way.
A map is available from the tourist information office (called the VVV in the Netherlands). Sluis is a sizeable town that is easily reached using the Brugge-Breskens bus, which is number 42 when run by the Belgian company De Lijn but number 2 when run by the Dutch company Connexxion. Alternatively, there are plenty of cycle routes in the area, so it is equally possible to cycle there.
The remains Lines of Fontaine-Cantelmo and Fort Isabella are within easy cycling distance (go along the main road out of Sluis towards Westkapelle and the road corsses the lines). The nearby fortification of Retranchement is also worth a visit.
To get to Retranchement, which is about 2.5 miles (4km) from Sluis, take the Cadsand road and turn left before the canal in a car or after the canal for the cycle track.