Even though there are some documents between the end of the twelfth and fourteenth centuries that mention a place called Sassofortino, which is distinct from the castle of Sassoforte, material analysis rules out the possibility of the castle being older than the 1400s.
The Comune di Siena attempted to populate the area in 1339 with a willing group of men who had once inhabited the castle of Sassoforte, but this seems to have ended in terrible failure, largely on account of the plague that devastated all Europe halfway through the fourteenth century; the two sites were registered as abandoned around 1430. Only the regularity of the urban plan on which the historic centre is designed might reveal traces of this first Sienese intervention.
The architectural remains cannot date back further than the second half of the fourteenth century, a date to which historical sources attribute a late agreement of population between the Comune di Siena and a group of men originating from Imola and Faenza.