Integrated GPR surveys and 3D modelling of the Sforza Castle’s underground network

The research on Castello Sforzesco in Milan developed an integrated 3D information model combining historical sources and contemporary survey technologies. A digital twin was structured through multiple Levels of Geometric Information (LOGI), organized by attributes and accuracy of the original information. The system was conceived not only as a repository of knowledge, but also as a dynamic analytical environment capable of stimulating new research questions and supporting further investigations. This potential led to collaboration between the Politecnico and Codevintec Italiana s.r.l., resulting in a wide survey with ground-penetrating radar. The Ground Penetrating Radar (GPR) investigations conducted within the Milanese fortress area, which ultimately led to significant discoveries, were themselves the result of studying this model. Extensive investigations were carried out using stepped-frequency array and single-channel impulse GPR systems, enabling high-resolution mapping of the castle’s underground infrastructure and surrounding defensive structures. The results revealed an unexpectedly complex subterranean network, including buried corridors, connecting tunnels, and a “twin” counterscarp gallery, drawn by Leonardo da Vinci and now located a few centimeters from the currently accessible one. The surveys also provided significant physical evidence supporting the existence of the legendary underground passage between the castle and Santa Maria delle Grazie. The results obtained have demonstrated the effectiveness of the multidisciplinary approach for the exploration of complex heritage sites. The research highlights how digital twins enriched with non-invasive diagnostic data can evolve from simple static representations into powerful interpretative and decision-support tools for the conservation, analysis, and enhancement of cultural heritage.