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.