The hinterland of Rimini is characterized by the presence of many castles, but the fortress of San Leo is certainly the most representative because of its position and the different constructive contributions that over time have updated its appearance and military functions. Cited by Dante and Machiavelli for the impervious nature on which it stands, its origin dates back to the early Middle Ages. It was rehashed following the imprint of Francesco di Giorgio Martini in the fifteenth century, restored by Giuseppe Valadier at the end of the eighteenth century and converted to a prison in 1631.

A peculiarity that makes the studies on the fortress of San Leo absolutely interesting is the treatment of the external architectural surfaces of which there is ample documentation in the historical archives and of which there are multiple uses in the various areas of the factory; the research aims to offer useful knowledge for the subsequent conservation and restoration project. The theme, completely original, arises from indirect investigations of a documentary and iconographic nature, conducted at the State Archives of Pesaro, Florence, Rome, the Central State Archive and the Vatican Secret Archive, which repeatedly refer in the accounting of works, starting from the seventeenth century, the execution of plasters executed outside the monument. The interpretative tension of the archival documents and the drawings continued by looking for a direct comparison between historical information and materiality of the fortress, in order to identify a correspondence between historical data and constructive reality.

It emerges clearly that the external surfaces of many parts of the fortress were treated and finished with plaster since its origins, probably due to the exposure to atmospheric agents; therefore a rethinking of what is reported in the literature is necessary both in terms of interpretative profile of the fortress, and about how its image was perceived over the centuries.