Leonardo da Vinci was not only a master of art and science but also an expert in military engineering, pioneering innovative warfare strategies during the Renaissance with his designs and inventions.
Recently, scientists identified a concealed structure beneath a medieval castle in Milan, Italy—an underground passageway that was previously only hypothesized based on a 1495 sketch by Leonardo and references in historical texts. These tunnels were likely designed for soldiers to use if the castle’s defenses were breached.
The discovery, announced by the Politecnico di Milano in January, was made using advanced, non-invasive techniques such as ground-penetrating radar and laser scanning to digitally map the 15th-century Sforza Castle’s underground architecture.
The surveys, conducted between 2021 and 2023, originated as a doctoral thesis by Francesca Biolo, who is now an architect and research fellow in the Politecnico di Milano’s department of architecture, built environment, and construction engineering.
“Our discoveries are yet another testament to the rich history woven into our cities,” Biolo stated in an email. “Only by recognizing this and deepening our understanding of history and architecture can we truly value the importance of preserving and enhancing our cultural and architectural heritage.”
While Leonardo da Vinci’s exact role in the castle’s construction remains uncertain, the findings “further highlight the impact of this great thinker on the history and architectural evolution of Sforza Castle,” she added.
Initially, Biolo and her team set out to digitally map the underground area beneath the Ghirlanda, the castle’s outer defensive wall. This area includes a well-documented underground passage along the moat, which is accessible to tourists. However, their survey unexpectedly revealed a second, previously unknown tunnel that had long been speculated about by experts.
This newly discovered tunnel runs parallel to the first, approximately 1 meter (3 feet) below the surface. According to Biolo, it was likely used by soldiers to covertly defend and reclaim the Ghirlanda in case of an enemy attack.
The research team also uncovered additional tunnels—mostly constructed of brick with barrel-vaulted ceilings—including one leading toward the Basilica of Santa Maria delle Grazie, where the wife of Duke Ludovico Sforza is buried. Also known as Ludovico il Moro, Sforza ruled Milan from 1494 to 1498.
Historical records suggest that the grief-stricken Duke Ludovico Sforza may have commissioned the tunnel to provide easy access to his late wife’s resting place, according to the Politecnico di Milano. However, Biolo noted that further surveys would be necessary to determine whether the tunnel indeed connects to the Basilica of Santa Maria delle Grazie.
In addition to the tunnel, researchers detected medium-sized underground rooms at a second subterranean level, she added.
Biolo and her team believe that more hidden structures remain undiscovered beneath the castle. However, mapping the entire underground network is currently not feasible. She explained that the original castle was about six times larger than its present form, with the visible structure today covering approximately 40,000 square meters (430,560 square feet).
Over the centuries, the castle underwent significant demolitions and restorations, particularly during the Napoleonic wars and the late 19th century. “These new discoveries show that not everything was lost,” Biolo said. “The underground preserves traces of our past—not just from this period, but from many others.”
During the late 1400s, Leonardo da Vinci frequently spent time at Sforza Castle as part of Duke Ludovico Sforza’s court. The duke commissioned him to create a painting for the castle’s Sala delle Asse, or “room of wooden boards.”
During this time, Leonardo sketched designs of defensive structures that closely resemble the layout of Sforza Castle, including the Ghirlanda and its network of passageways.
Dr. Francesca Fiorani, an art history professor and Leonardo da Vinci expert at the University of Virginia, stressed the importance of reconstructing the past with as much accuracy as possible.
“In Leonardo’s case, most of his architectural drawings were conceptual exercises—visions for innovative structures rather than blueprints for actual construction,” Fiorani explained. “One might call it ‘paper architecture’ that existed solely in sketches.”
She noted that Leonardo’s drawing of the passageways beneath Sforza Castle likely reflected modifications he was considering, as some details in the sketch differ from the castle’s actual layout. However, she emphasized that further research is needed to determine whether his designs directly correspond to the newly discovered tunnels.
Biolo agreed that while there is a “clear connection between the elements in Leonardo’s drawings and the actual structures,” it remains uncertain how directly involved he was in the fortress’s construction.
Regardless, the discovery underscores how history can be found in the most unexpected places, Biolo said. She is currently involved in a project focused on preserving and enhancing municipal buildings in various Italian towns.
“Perhaps the most important goal for the future is not necessarily another groundbreaking discovery,” Biolo reflected, “but rather a growing awareness and commitment to preserving our heritage—especially the parts often overlooked—using the vast possibilities that today’s knowledge and technology provide.”
