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Leonardo da Vinci excelled in the fields of art and science, but the Italian polymath was also a highly skilled expert in military structures and defensive systems, breaking new ground in the strategy of warfare during the Renaissance with his designs and inventions.
Now, scientists have uncovered a hidden feature underneath a medieval castle in Milan, Italy, that researchers once could only speculate about based on a sketch of Leonardo’s from around 1495 and references in other historical sources — underground passageways that were likely intended for soldiers to use in the event that the castle’s defenses had been breached.
The discovery, which the Politecnico di Milano announced in January, came about through a series of surveys that aimed to digitize the 15th century Sforza Castle’s underground structures through nondestructive methods such as ground-penetrating radar and laser scanning.
The surveys, which ran from 2021 to 2023, began as a doctoral thesis for Francesca Biolo, who is now an architect and research fellow in the department of architecture, built environment and construction engineering at the Politecnico di Milano.
“Our findings serve as yet another reminder of how deeply embedded history is within our cities,” Biolo said in an email. “Only through awareness of this fact, combined with a thorough understanding of history and architecture, can we truly appreciate the importance of preserving and enhancing our cultural and architectural heritage.”
The extent of Leonardo’s involvement in the construction of the castle is unclear, but the finding “reinforces the influence of this great thinker on the history and architectural development of the Sforza Castle,” she added.
Biolo and her team originally intended to digitize the area underneath the Ghirlanda, a protective outer wall of the castle. It features a well-known underground passageway that runs along the perimeter of the castle’s moat and is accessible to tourists. But to the researchers’ surprise, their survey revealed a second secret tunnel that experts had only hypothesized about for years.
The second tunnel runs parallel to the first one, about 1 meter (3 feet) beneath the surface. It is believed that soldiers would have used the concealed route to defend and reclaim the Ghirlanda against enemy forces, Biolo said.
The researchers also uncovered other tunnels — almost all made of brick with a barrel vault, or curved ceiling — including one that heads in the direction of the Basilica of Santa Maria delle Grazie, the resting place of the wife of Duke Ludovico Sforza. Also known as Ludovico il Moro, Sforza was the duke of Milan from 1494 to 1498.
Historical sources indicate the grief-stricken duke would have built the tunnel for easy access to reach his late wife, according to the Politecnico di Milano. More surveys would be needed to confirm whether the tunnel connects to the basilica, Biolo said. In addition to the tunnel, medium-size underground rooms were also detected at a second subterranean level, she added.
Researchers suspect that there are more subsurface structures to be uncovered, although it’s still not feasible to map everything underground, Biolo said. The castle once spanned an area about six times the size of what stands today — the footprint of the building that is currently visible above ground is around 40,000 square meters (430,560 square feet), she added.
The castle went through extensive demolitions and restorations during the Napoleonic wars and at the end of the 19th century. “These new findings, however, reveal that not everything was lost,” Biolo said. “The underground holds (traces) of our past — not just from this era, but from many others.”
Leonardo often spent time at the castle during the late 1400s as a member of the court of Duke Ludovico Sforza, who commissioned the artist for a painting featured in the property’s Sala delle Asse, or room of wooden boards. During this period, Leonardo produced the drawings of defensive structures that closely resemble the layout of the Sforza Castle, including the Ghirlanda and multiple passage systems.
“It is always important to be able to reconstruct the past as precisely and as firmly as possible,” said Dr. Francesca Fiorani, an art history professor and resident expert on Leonardo at the University of Virginia in Charlottesville. “In the case of Leonardo, we know that most of his drawings, especially the architectural drawings, were ‘mental’ exercises, ideas for innovative buildings, but that they were not meant as blueprint for actual construction, just ideas of innovative building: paper architecture that existed only as drawings on paper, one might say.”
The sketch of the passageways underneath the castle most likely depicted some improvements the artist was exploring for the Sforza Castle since he drew some features differently from how the castle actually looked, added Fiorani, who was not involved with the recent discovery.
“Whether his drawings and the tunnels recently discovered correspond (is) a matter (that) needs to be determined with further investigation,” Fiorani said in an email.
There is a “clear connection between the elements depicted in his drawings and the actual structures,” but at this time it is “impossible to determine the extent of Leonardo’s direct involvement in the fortress’s construction,” Biolo said.
Nevertheless, the discovery highlights the notion that history is everywhere, even in places where it is least expected, she said.
Biolo is currently involved in a project that looks at the enhancement and preservation of some municipal buildings in other Italian towns.
“Perhaps what I hope for most in the future is not so much a new and groundbreaking discovery,” Biolo said, “but rather a growing, widespread, and conscious tendency towards the preservation of our heritage — especially that which is often forgotten — supported by the vast possibilities that today’s knowledge and technologies offer.”