Sluis received city rights in 1290, riding the long medieval boom of the Zwin inlet, a tidal arm of the North Sea that punched deep into Flanders and made Bruges and its outport Damme into commercial giants. Sluis sat near the seaward end of this funnel, ideally placed to handle the overflow trade. By the early fourteenth century it was one of the busiest harbors in Christendom, a Hanseatic stop where Baltic timber traded for Flemish cloth and Mediterranean wine. The town's most famous appearance in English textbooks came in June 1340, when Edward III's fleet annihilated the French navy in the Zwin estuary at the Battle of Sluys, an opening victory of the Hundred Years' War that gave England command of the Channel and made it possible to land armies at Calais a generation later.

Sluis kept records that show how thoroughly money ran the town. On 9 May 1445 it held one of the earliest documented cash lotteries in European history, paying out 1,737 florins, a sum worth roughly 170,000 US dollars in 2014 money. The belfry that still dominates the skyline dates to the same age of confidence. Inside it today hangs Andries Eertveld's painting of the 1603 naval battle, in which a Dutch squadron under Joos de Moor crippled a Spanish galley fleet just offshore and killed the Italian captain Federico Spinola. By that point the Zwin was already silting; another generation and the harbor would be unusable for serious shipping.

The Eighty Years' War left its marks. In 1587 the Duke of Parma took Sluis for Spain. In 1604 Maurice of Nassau, supported by English troops, took it back. By then the deep-water trade had already moved to Antwerp and Amsterdam, and the silt was winning the war that mattered. Centuries of land reclamation, Dutch dike building, and the natural settling of suspended sediment turned the Zwin from a funnel into a finger and then into a dry channel. The sea, the engine of the town's prosperity, slowly and quietly walked away. Sluis is now closer to the Belgian border than to open water. The Zwin survives as a small nature reserve favored by herons and birdwatchers.

A town that stops growing produces a specific kind of biography: people leave, do something somewhere else, and Sluis claims them later. John Crabbe, born before 1305, made a career as a Flemish merchant, pirate, and soldier whose loyalty followed the highest bidder. Jacob van Loo, born in Sluis in 1614, painted figures of the Dutch Golden Age. Johanna Jacoba van Beaumont, born around 1752, became a journalist, feminist, and editor in a century that rarely allowed women to be any of those. Johan Hendrik van Dale, born in Sluis in 1828, became the lexicographer behind what is still the most authoritative Dutch dictionary; ask anyone in the Netherlands what 'de Van Dale' is and they will not ask you to specify which Van Dale. The town's most quietly impressive recent achievement was the restaurant Oud Sluis, which from 2005 to 2013 held three Michelin stars, one of only two in the entire Netherlands.

One kilometer west of Sluis sits Sint Anna ter Muiden, a village with a 2001 population of fifty. It also happens to be the westernmost inhabited point of the Netherlands, a fact the residents seem content to keep mostly to themselves. The whole municipality, including Aardenburg, Oostburg, and a scatter of smaller communities, holds about 23,000 people today. Tourists come for the belfry, the Wednesday market, the cycling along old dike roads, and Belgian day-trippers come for the Dutch shops that open on Sundays when their own are closed. A ferry between Breskens and Vlissingen still runs across the Westerschelde, but only for pedestrians and bicycles since the Westerschelde tunnel opened near Terneuzen in 2003. The car traffic, like the ocean traffic before it, has moved on. Sluis remains, watchful and intact, the bell ringing the hours over fields that used to be sea.