The structure of the fortress is made up of two fundamental elements of geometric organisation: a multi-sided core, that of the early fortress, with typical defensive constructions, and a more recent interior, that of the more regular-shaped, and smaller, Palazzo Del Monte. The two elements are connected, near the historic centre, by a mighty wall containing walkways and, to the north-east, by a covered balcony at the height of the “piano nobile” (the main floor of an Italian Renaissance building)
An analysis of the structure and dimensions of the building supports the hypothesis that the fortress was originally limited to the multi-sided structure of the fortress and perhaps also the external defensive wall.
It includes all the structural characteristics of a building clearly suited to defensive functions: a multisided floorplan with towers located in every intersection of the fortress walls. The two tall and imposing towers- the central “Maschio” and the “Torre Cavaliera” – are positioned diagonally from each other, and two others emerge from the uninterrupted walls that surround the fort. The one facing east is easily identifiable. We can only hypothesise about the other, where the chapel of Saint John the Baptist is located.
It shows an expansive elevation with strong vertical lines and very few windows. Those windows which are present are mostly on the upper floors. It should be noted that the large windows on the “piano nobile” are nothing more than the result of an adaptation in the seventeenth century. In a period in which the need for a strong focus on defence had abated, the crude fortress was transformed into an aristocratic residence by the then papal legate Cardinal Salviati.
In addition, external interconnecting passageways and stairs between the various floors were constructed of wood. A floorplan of this type supports the hypothesis that the current internal trapezoidal staircase was inserted into the space of the original courtyard of the fortress, in the centre of which there was already a water reservoir fed by the aqueduct coming from the springs of Santo Marzio.