Inside, the fortress houses a considerable amount of works of art and tools of use linked to its past: in the main tower, accessible from the large Piazza d'Armi, there are cannons, halberds, crossbows and armor; the small north tower is the result of a nineteenth-century reconstruction - designed by the architect Giuseppe Valadier - following the collapse of two pre-existing towers, in turn caused by the collapse of the supporting cliff. From the Piazza d'Armi, walking up a flight of steps and passing a Gothic portal, you reach the courtyard which gives access to the medieval keep and the ducal residence. The ground floor of the sixteenth-century building houses further movable assets ranging from archaeological finds to period prints. The main floor includes, in typically Renaissance architecture spaces, rich furniture from various periods. The second floor, which once housed the prisoners first and then the military garrisons, is now used as an exhibition space (the room dedicated to the collection of twentieth-century weapons is remarkable for its completeness and quality). Perhaps the most famous and visited place of the fortress is the so-called "pozzetto", that is the cell located in one of the three square towers of the keep and where the alchemist Cagliostro was imprisoned for four years, condemned for heresy by the Holy Inquisition and died right in San Leo in 1795.