‪Last year‪ marked the 500th anniversary of Leonardo da Vinci’s death in Amboise, a town on the southern bank of the Loire River in France. In 1519, that very same year, construction began on a castle 50km east of Amboise, the Château de Chambord – the largest castle in the Loire Valley and widely considered one of the most impressive examples of French Renaissance and Medieval Revival architecture.

But whoever originally imagined the defining features of this great monument remains a mystery: Chambord was commissioned by King Francis I, but no architectural plan for it exists.

Some art historians think the clue to the castle’s design may lie in Chambord’s most iconic feature: its unusual “double helix staircase”, which allows two people to ascend without ever having to cross paths (perfect for steering clear of servants or irritating relatives). Composed of two helical ramps that twist like a strand of DNA around a hollow core, the staircase services the main floors of the castle all the way up to the crowning terraces. The staircase’s design – never before seen in France – seems to suggest a more than coincidental link with the famous Italian polymath, whose notebooks were filled with similar architectural sketches and designs on everything from plumbing to horticulture.

To some, Leonardo’s influence on Chambord is undeniable: its ornamental facades, modular interiors and grid-style layout bear his hallmark elegance and visionary logic; indeed the castle carefully promotes itself as a place “imbued with the spirit of Leonardo”. But it does not claim him as the chateau’s architect, because of a lack of proof. At the quincentenary of the artist’s death – and the castle’s inception – scholars and visitors continue to be ensorcelled by these two enigmas.

While Chambord’s elusive paternity remains a divisive subject, a few archival breadcrumbs scattered throughout the centuries reveal Leonardo’s influence at work: a lost wooden model; a hidden sewer system and royal visions of a fairytale castle, its turrets twisting up toward the heavens like a crown.

The original Chambord?

In 1516, Leonardo left his studio in Rome to join the court of King Francis I as “premier peintre et ingénieur et architecte du Roi” (“the King’s first painter, engineer and architect”). Leonardo had met the glamorous young Francis one year earlier in Bologna, and, tempted by the offer of a handsome pension and the new king’s patronage, travelled to Château du Clos Lucé in Amboise, where he would spend the remainder of his days. Leonardo accepted few commissions while in France, perhaps because he was preoccupied with a more ambitious pursuit: designing a royal residence the size of a small city which would showcase revolutionary feats of engineering both practical and decorative.

At the time of Leonardo’s arrival in France, the king was busy with plans for an opulent royal residence, one that would serve as an idyllic palace and a testament to the legitimacy of the Valois dynasty. From paintings to rare books and manuscripts to fortresses, Francis I enthusiastically embraced the cultural Renaissance that had swept Italy, eager to put his imprimatur on the arts, and in 1516 commissioned plans for his dream castle at the site of Romorantin (near present-day Blois, about 80km from Amboise). For Leonardo, it was an ideal assignment – the culmination of an illustrious career, allowing the artist to express many of his passions: architecture, urban planning, hydraulics and engineering. Ideas Leonardo had first developed in Milan might now be realised in physical form.

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“Beginning in the 1490s, Leonardo was teaching himself to be an architect and engineer,” said Pascal Brioist, science historian at Tours University, whose book The Audacity of Leonardo da Vinci explores the connections between Leonardo and Chambord. “Six of his drawings show a project for a French palace, drawn before Chambord was even conceived.” The drawings that Briost is referring to are in Leonardo’s Codex Atlanticus, which feature images of twisting staircases, helical towers, complex plumbing and elaborate gardens – many of which can be seen at Chambord.

But Leonardo didn’t label his drawings as corresponding to any particular castle plan, and he died four months before construction began on Chambord. Were these sketches part of a design for Romorantin? Or were they somehow evidence that Leonardo was the mastermind behind the great Chambord?

Construction on Romarantin did begin – a 400m terrace was even built above the Sauldre River in preparation for a palace with stables and royal accommodations – but, perhaps because of Leonardo’s illness and death, or because Chambord proved a better location, the project was abandoned shortly after earthwork began. Whether Leonardo was the mastermind behind the great Chambord remains a mystery – one seemingly buried under the rubble of Romarantin until more than a century later.

An elusive architect

As a work of architecture and an affirmation of royal power, Chambord is radically unique: a place that conceals almost as much as it reveals. The chateau is a Unesco World Heritage site with more than 400 rooms, including reception halls, kitchens, lapidary rooms and royal apartments. Originally conceived as a hunting lodge, Chambord boasts a fireplace for every day of the year. Its stone terraces offer sweeping views of the gardens, stables and, in the distance, a vast game preserve teeming with wild deer and boar.

Within its gilded expanse are more impressive features. Inside Chambord’s two great wings – spread over four floors in the shape of a Greek cross, with staircases that lead up to dramatic lantern cupolas – caissons and vaulted ceilings are decorated with an assortment of royal insignia, including salamanders, porcupines, dolphins and other inscrutable icons.

The model is the missing link to Chambord

More than 100 years after the palace was completed, a discovery by André Félibien, court historian to King Louis XIV, shed new light on the link between Chambord and Leonardo. While rummaging in an attic in Blois in 1681, Félibien chanced upon a wooden model of a castle – an early model of Chambord, according to Brioist. Although the model did not survive, Brioist argues that it’s a definitive clue linking Leonardo to the king’s new project at Chambord, because it bore similarities to Leonardo’s initial sketches of castles in the Codex Atlanticus, with pinwheel floorplans, a central keep in the shape of a cross and helical structures. In other words, the model shows that Leonardo’s ideas were likely used to shape the future Chambord.

“The model is the missing link [to Chambord],” Brioist said.

However, the model was likely built by Domenico da Cortona, a well-known architect summoned from Italy to the French court during the reign of Charles VIII (1483-1498). Indeed, royal accounts note a payment in 1532 of £900 to Domenico for his wooden model of the chateau. “Leonardo physically cannot be Chambord’s architect,” said Brioist, “but his ideas infused the castle through the influence of Domenico da Cortona and the king.”

“Maybe the architect was Leonardo, maybe it was Domenico da Cortona,” said Virginie Berdal, research fellow at the National Estate of Chambord, explaining that during the 16th Century, the architect’s role was far from the occupation as we know it today: architects would have provided conceptual ideas for buildings, but not necessarily draw up blueprints. Thus, although it’s possible that Leonardo came up with the architectural concepts behind Chambord; the man who designed the castle would have most likely been Domenico da Cortona.

To shed light on the castle’s paternity, scholars turn to the archive. “We discovered that Leonardo da Vinci and Domenico da Cortona worked together during a feast at the Château d’Amboise,” said Brioist. The two men collaborated on plans for a sumptuous royal feast celebrating the birth of the Dauphin in 1518, which certainly could have allowed de Cortona an opportunity to pick Leonardo’s brain about plans for a chateau.

One year after the feast, a new project would replace Romorantin: masons would lay the first stones at Chambord, and Leonardo would be dead, buried in his tomb at the Château d’Amboise.

An ongoing detective story

A recent exhibition marking the chateau’s 500th anniversary, “From Utopia to Action,” explored the possibility that Chambord was the Italian master’s last architectural creation, taking up the centuries’ old debate once again. “Notwithstanding the extraordinary renown of the chateau and the interest it has aroused among historians over the centuries, the identity of Chambord’s architect has remained an enigma,” said Berdal.

She emphasised, however, that similarities between Chambord’s highly unusual design and Leonardo’s castle drawings in the Codex Atlanticus are too conspicuous to ignore. “The Greek cross-shaped centre-plan design of the chateau, a large-scale centrally located copula, the exact proportions and modular layout and a double-helix spiral staircase [are] remarkably connected with the studies drawn by Leonardo da Vinci.”

The identity of Chambord’s architect has remained an enigma

While the model, the codex drawings, and financial record are compelling clues, recent archaeological breakthroughs provide even more evidence to suggest that Chambord was a creation of Leonardo’s. In the 1990s, archaeologists discovered a vast network of sewers underneath Chambord. “The latrine system set up in Chambord’s keep is singularly sophisticated,” said Berdal. Unsurprisingly, its layout, with masonry ducts that allow for ventilation, corresponds to recommendations Leonardo made in his notebooks for just such a system.

A decade later, in the 2000s, experts were able to use electrical prospecting to discover an anomaly in the orientation of the towers: the southern, eastern and western towers are all identical, but the northern tower had been rotated 90 degrees in respect to the centre keep. “A tower has changed direction,” Brioist explained. “The stones are arranged in such a way that we think there was once intended a helical structure, twisting like a crown.”

Leonardo was famously preoccupied by vortexes, gyroscopes and whirlpools, as well as by central symmetry and aerology. The idea that Chambord was meant to be one perpetually twisting structure, its four towers continuing the axis of movement around the staircase – almost like a pinwheel – suggests Leonardo’s influence was at work. Berdal agrees: “These observations indicate that in the original design, the Chambord keep was gyratory; that is to say moving in a circle or a spiral.” Even if engineers eventually abandoned this plan (probably because King Francis decided to add two additional wings in 1526 to accommodate more visitors), Leonardo had left his mark.

“Whether by dint of the artist’s spirit or his hands,” said Berdal, “a connection with Chambord is indubitable. Chambord is ultimately a Vincian dream.”

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