The recovery of the “mastio” and the surrounding courtyard of the new fortress of Volterra (1472-1474) has as its objective the opening to the public of this fortified work, after 542 years from its construction, with the possibility of being enjoyed without interposing with the prison function of the complex, it also represents the possibility of knowledge and study of a constructive typology in the context of the Renaissance fortified architecture of the Italian school called “transition” with the use of the first artillery. The fortress was born as a military garrison and at the time of Lorenzo the magnificent only a part was used as a prison, it will be definitively transformed into a House of Imprisonment during the Grand Duchy of Lorraine in the middle of the eighteenth century. It is the first work by Francesco di Giovanni di Matteo called the Francione (1428-1495), it has an almost square shape with large cylindrical towers at the corners (rondelle) and at the center of the inner courtyard, a large cylindrical tower like of “mastio” (donjon) and inserted the artillery in the walls. The “mastio” consists of a basement and five floors above ground with a domed roof and connected by a narrow spiral staircase. After the cognitive essays carried out on the internal domes of the “mastio”, placed in the first three floors including the cistern, the presence of “hemispherical domes” emerged, made by workers of the Opera del Duomo in Florence, built entirely in bricks without the carpentry of “centina” (self-supporting), with the system called “alla fiorentina”, as well as the dome of Santa Maria del Fiore in Florence by Filippo Brunelleschi. This construction system is also applied in the fortified structures of Pietrasanta, Poggibonsi, Sarzanello, Castrocaro, Pisa and Terra del Sole.