The old medieval structure, whose real consistence is difficult to estimate, was demolished in 1370 following the wars that broke out in that year when Castruccio Anteminelli's heirs tried to recapture the Guelph territories that had submitted to Lucca.
When Sassi, like the overwhelming majority of communities in Garfagnana
, became part of the Duke of Este's dominions, the fort was rebuilt and expanded as it came to lie right next to the border with the Republic of Lucca and its nearby Gallicano, Perpoli
and Fiattone possessions; it so became, together with Trassilico
, one of the garrisons guarding the southern frontier.
Moreover, its proximity to Castelnuovo
, capital of the Province of Garfagnana and exposed to raids from Lucca because it only had an old circle of walls built by Castruccio to defend it, made Sassi the ideal refuge for the town's inhabitants in case of siege. The Dukes of Este maintained a garrison here, quite modest to tell the truth, as a list of the supplies and arms for the year 1488 proves.
During the Florentine invasion of Garfagnana in 1512 the rocca
was besieged and taken by mercenaries paid by the Republic of Lucca in an attempt to take advantage of the weakness of the Dukes of Este, involved as they were in a war against Pope Leo Xth
of the Medici family; however, as soon as the outcome of the war became clear, the fort was handed back to its legitimate owners.
With the beginning of the XVIth
century the relentless decline of the rocca started: it was abandoned and in 1537 Duke Ercole granted it to one of his subjects, Jacopo di Mariano of Sassi, in recognition of the courage shown by his father during the siege of Lucca's troops in 1512, provided that, if the necessity arose, he could always use it to shelter men and store provisions. As a consequence the structure began to decay progressively and we know that by the first half of the 17th
century it had lost all military and defensive functions and its grounds had been simply converted into fields.
After the collapse of the bell tower, originally incorporated in the church, at the beginning of the 18th
century a new one was built not much further.