The drawing of a fortress dates back to Leonardo’s second Milanese period, when the Sforza had been expelled from the city and the French had taken control. This project was probably prepared for the French government, at a time when military defence was of paramount importance.

Leonardo had already provided military designs for Ludovico Sforza in Milan, the Venetian authorities and Cesare Borgia, following Cesare in his campaign to conquer the Romagna region. This allowed him to address an important innovation in warfare of the period: firearms, and how to plan a defence against them.

This fortress can be considered very “modern” in its design: all the surrounding walls are slightly inclined and the towers are circular. This served to better absorb the attacks from firearms. Another innovation was the absence of traditional battlements: the tops of the walls are rounded, which helps deflect the impact of cannon fire.

Small openings allow the defenders to return fire. All thirteen towers are provided with a round covering and the fortress features two levels of concentric walls. The central portion of the complex is where the lord of the castle lived. Leonardo’s drawing also shows a secret underground passage.