After the fall of the Roman Empire, Cassel became an important fortified stronghold for the rulers of Flanders which was repeatedly fought over before finally being annexed to France in the 17th century.

It is renowned for its extensive views from the summit of Mont Cassel and is the location of the Nord department's principal museum of local art, history and folklore.

[4] The hill of Mont Cassel was occupied during the late Iron Age by the Menapii, a Belgic tribe, who made it the capital of a large territory extending from modern Calais to as far as the Rhine.

Cassel was redeveloped as Castellum Menapiorum, the urban centre or civitas of the Menapii;[7] the modern town takes its name from the Roman settlement.

From the 1st century AD onwards, Cassel developed into a key urban centre for the whole region with an extensive road network converging on the hill.

[8] Towards the end of the 3rd century, however, repeated invasions of Germanic tribes caused devastation throughout the region and at least 80% of settlements in the area are thought to have been abandoned, accompanied by a general economic decline.

The castle does not survive today—it was already in ruins by the early 18th century—but is depicted on old engravings as a large square tower, the Tour Grise, dominating the western flank of the hill.

The rebels had driven the ruling Count Louis I out of Flanders and sought to press their advantage by occupying Cassel and attacking the French royal army nearby.

The French repeatedly fought with Spain and later the independent Dutch Republic for control of the town;[citation needed] in March 1645, Gaston, Duke of Orléans, seized it but lost it again to the Spanish a few months later.

[citation needed] The battle took place after King Louis XIV of France besieged the Dutch-held town of Saint-Omer during the Franco-Dutch War.

William sent an army to relieve Saint-Omer but was defeated at the village of Zuytpeene just to the west of Cassel, losing 7,000–8,000 men killed or wounded and another 4,000 taken prisoner.

The annexation led to the town's defences being dismantled; it was considered indefensible, as it was within easy firing range of the nearby Mont des Récollets, and it would have cost far too much to fortify both hills.

While it is true that Prince Frederick, Duke of York and Albany, did command an army in the Flanders Campaign of 1793 and was probably in the vicinity, the link with Cassel is considered dubious as the eponymous nursery rhyme appears to have originated well before the Revolutionary Wars.

Benjamin Disraeli, later to become Prime Minister of the United Kingdom, stayed there for a month in September–October 1845 and wrote in a letter to his sister Sarah that he considered it "an extremely savage place; few of the inhabitants, & none of the humbler classes, talk French, there is no library, bookseller's shop, nor newspaper of any sort ...

It is quite French Flanders, their provisions come from Holland, the Hotel de Ville was built by the Spaniards, the carillons are perpetually sounding, & religion is supreme.

After scoring initial successes against the tanks of Panzer Regiment 11, which had made the mistake of advancing without infantry support, the British garrison was heavily attacked from the ground and the air by German forces.

The Jardin des Mont du Récollets provides expansive views over the plains of Flanders and beyond; on a clear day it is possible to see the North Sea, the English coast and the belfry of Bruges.

The building originally housed the "Noble Court" of the Lords of Cassel, who had authority over a swathe of territory from Ypres in modern Belgium to Saint-Omer.