The municipal territory is characterized by important archaeological sites belonging to the Neolithic age as well as to the Etruscan, the barbaric and the medieval age, but the real "birth" of the castello di Castelnuovo can be assumed to have taken place around the 11th century.

Around the year 1213, the social class of free men with Langobard ancestors succeeded in gaining power with the help of the mighty municipality of Volterra, under which protection economy and civil life developed visibly.

Castelnuovo and its community were the heart of battles and fights between the bishop-count and the free municipality of Volterra for the possession of its mineral resources (silver, brimstone, alum and vitriol), for nearly two centuries until they finally entered the Republic of Florence (repubblica Fiorentina) in 1429, following its sharp fights and suffering invasions and plundering at the hands of imperial armies and mercenary troops serving hostile towns.

At the end of the 15th century, Lorenzo dè Medici and his court of humanists chose the thermal establishment of Bagno al Morbo to pass long periods of regeneration and rest there. Given as a feud to the family of the Albizi of Florence in 1639, it had been rebuilt in 1776 by Pietro Leopoldo I, the great sovereign belonging to the Enlightenment who gave birth to the industrial and social growth of his territory.

As a destination for literates, geographers, scientists (Lucrezio, Plinio, Dante, Ugolino da Montecatini, Leandro Alberti, Marullo, Busching, Miller, Mascagni, Hoefer, Giovanni Targioni Tozzetti, Maria Curie and many more), it got a new phase of economic growth starting from the year 1818 thanks to the development of the borax industry induced by Francesco de Larderel – a development that characterized its history until our times.