Why did a Roman emperor build a lighthouse in a French port city?

Nature · Published 29 September 2026 Swipe more like this →

In short: Boulogne-sur-Mer has served as a strategic gateway for two millennia, from Roman invasions of Britain to the front lines of world wars. Once the primary port for the Roman Empire's Britannic fleet, this coastal city has been repeatedly reshaped by conflict, trade, and its enduring relationship with the sea.

Founded by the Celtic Boii and known to Romans as Gesoriacum, the city became the essential link between the Roman Empire and Britain after Claudius's invasion in 43 AD. Emperor Caligula ordered the construction of a massive lighthouse, the Tour d'Ordre, around 39 AD to support military ambitions. The site remained a naval hub until the late 3rd century, when the admiral Carausius rebelled, prompting a successful siege by the junior emperor Constantius Chlorus in 293 AD. By 310 AD, the settlement was renamed Bononia, a name shared with Bologna in Italy, likely reflecting its status as a refounded or relocated community.

Throughout the Middle Ages, Boulogne functioned as the capital of an eponymous county. Its strategic value made it a frequent target during the Hundred Years' War, with multiple English occupations occurring between the 14th and 16th centuries. The city's history is marked by such shifts in power, including the 1550 Peace of Boulogne, where France paid 400,000 crowns to reclaim the territory. Even in the 19th century, Napoleon Bonaparte utilized the port as a staging ground for his planned invasion of the United Kingdom in 1805, though the supremacy of the Royal Navy ultimately thwarted his ambitions.

The 20th century brought further devastation, particularly during the Second World War. In 1940, British forces attempted to defend the city against German panzer divisions, and in 1944, a massive Royal Air Force bombing raid involving 297 aircraft completely destroyed the harbour. Following the war, the city underwent significant reconstruction in the 1950s and 60s, replacing lost infrastructure with functional, brutalist architecture. Today, Boulogne-sur-Mer remains France's largest fishing port, balancing its ancient heritage—such as its 12th-century UNESCO-listed belfry—with its modern role as a center for marine science at the Nausicaá national sea centre.

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