“Were I a Roman Catholic, perhaps I should on this occasion vow to build a chapel to some saint; but as I am not, if I were to vow at all, it should be to build a lighthouse.” — Benjamin Franklin, July 1757

Since antiquity, lighted beacons have guided ships to port. The earliest lighthouses were controlled fires on hilltops that warned vessels that they were approaching land. Over time, these signals were powered by burning coal or oil lamps backed by mirrors, which could reach navigators further out to sea. But lamp power was no match for a dark and stormy night; over centuries, broken hulls and wind-whipped sails ran aground as ships’ captains and crew perished within, unable to spot the coastline before it was too late.

All that changed in the early 1820s, when a French physicist invented a new kind of lens: a ring of crystalline prisms arranged in a faceted dome that could reflect refracted light. Augustin Fresnel installed his creation in the Phare de Cordouan, a towering lighthouse situated in France’s Gironde estuary, about 100km north of Bordeaux. Suddenly, one lamp could illuminate the way for sailors many nautical miles out to sea.

You may also be interested in:

• The last lighthouse keeper of Capri

• How France created the metric system

• How Fiji changed the way we travel

The oldest operating lighthouse in France (construction began in the 16th Century, but beacons had existed there hundreds of years prior) and the world’s first to be built in the open sea, this imposing sentinel of white stone is a Renaissance masterpiece. Equal parts cathedral, fort and royal palace, this ‘Versailles of the Sea’ is a monument to history and maritime engineering. The spot was listed as an historic monument the same year as Paris’ Notre Dame by the French Ministry of Culture, in 1862. Accessible only by boat, the Cordouan lighthouse offers visitors a revolutionary view of France’s heritage: the chance to climb inside the upper reaches of an old lighthouse, and into one man’s imagination.

A spectacular showpiece

The Médoc Atlantique is a bountiful stretch of south-west France famous for its vineyards, wines and chateaux; few tourists venture north of Bordeaux to the sleepy town of St Palais-Sur-Mer. From this vantage, the Cordouan lighthouse is unmissable on its lonely promontory, though Palais’ residents seem only dimly aware of its faithful watch. Beachside cafes touting fresh fish and Nutella-filled crepes are popular with locals, and many will take a boat ride to trawl for Coquilles St Jacques and then explore the surrounding pine forests. But once a day, a catamaran departs from Port Royan, taking passengers out of the harbour and into open waters. As the town fades from view and the ship’s sails flap wildly in the wind, the lighthouse rises up, breaking the horizon like a pillar of cloud. Most cannot help but wonder: why build such a magnificent showpiece where few would ever see it?

The Cordouan lighthouse offers visitors a revolutionary view of France’s heritage

In fact, the Phare de Cordouan’s spectacular architecture is the result of a long and turbulent history. According to legend, small beacons had existed on the unnamed islet since the early 9th Century, when Charlemagne supposedly commanded a light be shone there. It’s more certain that the Black Prince (Edward of Wales) was the first to build an actual tower on the sandbar, in 1360. More than 200 years later, in 1584, King Henry III commissioned a lighthouse at the mouth of the Gironde. The king wanted an impressive tower worthy of his royal stature, one that would replace Edward’s crumbling edifice. He contracted famed Parisian architect Louis de Foix with orders to construct a ‘royal work’: a lighthouse with extravagantly decorated apartments, keepers’ quarters, a large lantern and its very own chapel.

Religious wars and various financial and technical challenges made for slow going, but de Foix carried on with his work, even after the death of Henry III (the king, as it happened, would never set foot in his lighthouse). By 1611, mariners who moored their vessels at the confluence of the Atlantic and the Gironde were greeted by a tremendous sight: granite steps leading to a 67.5m stone edifice, impervious to the crashing waves. Past the engineer’s apartment and keepers’ quarters, they would climb more than 300 steps through each of several airy galleries, whose interiors still reflect a style traditional for the period: gilt trim, ornate pediments; small alcoves and domed ceilings; black-and-white mosaic tiles. The ship’s captain might stop to pray for his vessel’s safe passage at the chapel Notre-Dame de Cordouan, before ascending past the salle des lampes to the gallery deck atop this most remote sentry by the sea.

The lighthouse was lit for the first time in 1611: a fire fuelled by tar, pitch and wood, sheltered by a lantern. In the mid-1640s, Cordouan’s light was blown off by a storm, so keepers replaced it with a lantern that burned whale oil and fixed it atop a metal basin; this allowed for greater control of the flame, but it also meant a dimmer glow, much to the chagrin of mariners. Whale oil was replaced in the early 18th Century by coal, which proved a difficult lighting system to maintain as keepers had to routinely haul fuel to the top of the lighthouse and stoke the fire day and night. In 1782, a system of oil lamps and copper lumières (or reflectors) was installed, which meant that lighthouse keepers no longer needed to tend to the fires. And by the end of the 18th Century, clockmakers devised a rotating light made possible by the same mechanisms that powered timepieces, resulting in the first revolving lighthouse lantern in the world. In calm weather, these oil lamps could adequately assist navigation, but they were not powerful enough to converge on a bone-soaked, storm-ravaged sailor miles from land.

Following the French Revolution of 1789, attempts were made to eradicate every symbol of the Ancien Régime, the feudal political system in France that held everyone a subject of the king. That meant a complete obliteration of all royal effigies and inscriptions inside Cordouan, which was itself a tribute to the monarchy, after all. Only the bust of Louis de Foix in the entrance niche of the lower room was left intact – it was simply too heavy to transport – and visitors can still see this statue today. At the same time, large-scale works were underway at the lighthouse aimed at improving the structure’s utility and the mighty reach of its beacon.

An ingenious invention

In the 19th Century, optics was an emerging field. Scientists were familiar with Dutch physicist Christiaan Huygens’ theory of light, which held that light travelled in the form of a wave, but many remained sceptical. Augustin Fresnel would effectively prove Huygens’ theory: by arranging a series of small, convex prisms into the shape of a beehive, the Frenchman discovered that he could capture and refract oblique light.

Fresnel’s system was based on a key principle of geometric optics: when light passes from one medium to another – for example, air to glass and then to air once again – it changes direction. The lens’ concentric arrangement and ‘bending’ of light created a combined light intensity much greater than the light source itself. This in turn allowed the light to be visible over greater distances. Fresnel installed his invention at Cordouan, a lighthouse already prominent in his home country, in an area known for its rugged coastlines and treacherous swells. By the 1860s, thousands of lighthouses had been fitted with Fresnel lenses, from small harbour lanterns to great sea lights.

Every lighthouse has its own signature

“Every lighthouse has its own signature,” said Mickael Neveu, one of four lighthouse guardians who live year-round at Cordouan (guardians take turns in pairs, for a fortnight at a time). He gestured towards the Fresnel apparatus inside the tower, which is restricted to keepers and custodians. “The light here occults three times every 12 seconds.”

An occulting light is a rhythmic light in which the duration of light in each period is longer than the total duration of darkness: in other words, it has the appearance of flashing off, rather than flashing on. As Cordouan’s lens rotates, its bulls-eye panels create beams of concentrated light, which successively pass into view of the mariner: to the south, a flash of red, to the west, green or white light, followed by darkness. These different colours direct maritime traffic according vessel size: the green sector indicates the main passage of the estuary, used by high-tonnage commercial shipping; the south passage, marked by the red sector, is used by vessels of shallower draught.

The Phare de Cordouan is the last lighthouse in France to be continuously inhabited by keepers. Visits are permitted for the public from April to November; the rest of the year, keepers ensure that the beacon shines day and night, while engaging in daily maintenance and upkeep of both the tower and the surrounding islet, which becomes entirely submerged at high tide. The Phare de Cordouan was officially submitted as a candidate for Unesco World Heritage status in 2002, a feat owed, in no small part, to the stewards who watch over it faithfully.

The solitary existence of lighthouse keepers has long captured the imagination: the stoicism of their ceaseless vigil; the integral role they play in buoyage and beaconage; the loved ones they leave behind on dry land. Benoît Jenouvrier has been a lighthouse guardian at Cordouan for eight years. He has sandy dreadlocks, a slightly weathered smile and a purposeful manner. When asked about the most difficult aspect of his job, Jenouvrier replied that it’s different for each guardian. “The weather conditions are difficult, the daily tasks, the winters…” Here he trailed off, as if weighing the responsibility of caring for something so formidable. “In fact, I am more at ease in winter, when there are no visitors here.”

Against the heavily barnacled base of the Phare de Cordouan, this solitary keeper was himself part of history – a long line of lighthouse guardians who have variously watched over a royal palace, a graveyard of shipwrecks, a site of scientific innovation and a place that forever altered ocean travel.

Places That Changed the World is a BBC Travel series looking into how a destination has made a significant impact on the entire planet.

Join more than three million BBC Travel fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter called "The Essential List". A handpicked selection of stories from BBC Future, Culture, Capital and Travel, delivered to your inbox every Friday.