Capalbio (GR) is located in the heart of the southern Maremma, along the border strip that in the second half of the fifteenth century marked the line between the Republic of Siena –became part of the Grand Duchy of Tuscany with the peace of Cateau-Cambrésis of 1559– and the Papal State. The historic center, built around the hill on which it stands, enclosed by a double circle of walls, emerges in the skyline of the surrounding landscape. The fortified structure of Capalbio has a non-simple construction history, especially for the remote phases, but that gradually becomes simpler from the second half of the sixteenth century. The reasons why the defence structure was built were exhausted in a relatively short period of time. The advent of firearms and the evolution of the tools and techniques to which the art of war used, as is well known, imposed a radical transformation of military architecture, which only in some cases, responding to a necessarily changed strategy, they were updated or completely renewed. The fortress of Capalbio was not part of the renovation program and this decision allowed the Maremma village to maintain its historic medieval core until the modern era. The results of this research derive from the identification and study of fifteenth century construction accounting documents, compared with the structures that still exist. It was thus possible to retrace the main construction and transformation phases of the fortified complex, identifying the period in which it was built. Finally, it is not by chance that in that context the fortress of the Rocca replicates a reiterated model, probably due to the widespread use of Ticino and itinerant Lombard workers, also documented on this site.