Wedged as it was into Lucca's territory, Trassilico represented in the XVth century the southernmost outpost of the Duchy of Este. The military architect Pasio Pasi who worked on several projects in Garfagnana supervised extension works on the fortress during the harsh conflict with the Republic of Lucca in 1613; these works were aimed at strengthening its defences and making it more suited to lodge heavy artillery. Unfavourable natural events caused its rapid decline beginning from the XVIIIth century when the podestà's residence was moved from the rocca to a building inside the village.