The Fortress of San Leo is a fortress and former prison in the Montefeltro region, remembered in connection with Cagliostro’s death and woven into the novel’s occult investigations. Its peculiar cell first gives Jacopo Belbo a reason to visit; the account he sends from there leads toward the Picatrix séance and the return of Colonel Ardenti’s six-seal message. Later, San Leo reappears in Casaubon’s feverish romance as the place where he is disfigured and imprisoned, making the fortress a setting where historical legend and imagined conspiracy collapse into one another.

The Fortress and Cagliostro’s Prison

In a letter to Casaubon, Belbo recalls driving through the Montefeltro area and happening upon the Fortress of San Leo. He explains that the region was under papal rule in the eighteenth century and that the pope imprisoned Cagliostro there. The cell was extraordinary in its arrangement: it had no ordinary door, and its occupant entered once and for all through a trapdoor in the ceiling. From a small window, the prisoner could see only the village’s two churches. The fortress is therefore introduced not simply as a landmark but as a place whose architecture makes confinement and restricted vision tangible.

Belbo reports that Cagliostro slept and died on a shelf in the cell. When Belbo visited, he saw a bunch of roses there, and was told that devotees continued to make pilgrimages to the site of Cagliostro’s martyrdom. The detail links a material remnant in the prison to an ongoing practice of veneration: the cell’s past is not sealed off as history, but is kept present through visitors who treat it as a sacred destination.

From Pilgrimage to the Picatrix Séance

Among the most assiduous visitors, Belbo learns, are members of Picatrix, a Milanese group of occult students that publishes a magazine also titled Picatrix. Curious about the group, he obtains an issue after returning to Milan and discovers an announcement for an imminent evocation of Cagliostro’s spirit. The fortress thus serves as the point of departure for a chain of investigation: devotion at the former prison leads Belbo to a modern occult circle and then to its attempted ritual.

At the séance, a woman enters a trance and speaks of a temple, a door that must be opened, a vortex of power, and an ascent to the Great Pyramid. The decisive connection for Belbo is her mention of six seals, a wait of one hundred and twenty years, and the thirty-six invisibles: he recognizes these as elements of the Provins message. After the woman collapses, Belbo tries to approach her, but Inspector De Angelis stops him and says that he knows where to find her. De Angelis explains that the police had traced Ardenti’s movements in Milan and, a year after his disappearance, learned by chance that someone had seen him leaving the Picatrix offices with the psychic woman. The fortress’s association with Cagliostro has therefore opened onto an unresolved police inquiry, rather than merely an antiquarian curiosity.

De Angelis questions Belbo about why the séance’s reference to six seals struck him so strongly. Belbo answers that Ardenti had mentioned a treasure protected by six seals, but says he did not consider the detail important at the time because treasures commonly have seals. De Angelis points out the tension between Belbo’s claimed indifference then and his sharp recognition now. Through this exchange, the fortress’s historical and devotional associations become entangled with the novel’s central problem of how fragments acquire significance: a detail that once seemed commonplace becomes charged when it resurfaces in a ritual and matches a cryptic message.

San Leo in Casaubon’s Imagined Romance

The fortress returns much later not as a place Casaubon visits, but as the destination of an elaborate, violent fantasy. In the imagined sequence, Casaubon casts himself as master of subterranean currents and lord of the six sacred places associated with the Thirty-six Invisibles. In a crypt at Tomar, a hidden chapel reveals Cagliostro and the avenging Luciano; the surrounding monks cast off their robes to reveal themselves as the Templars of Provins. They seize Casaubon and brand Baphomet’s sign onto his shoulder, telling him through their actions that he is to replace Cagliostro at San Leo.

The imagined punishment completes the fortress’s transformation from historical prison into a role in Casaubon’s own romance. To make the substitution possible, his captors thrust his face into a basin of vitriol. When he is pulled out, his face is unrecognizable; he imagines being returned to the dungeon like fugitives who disfigure themselves to avoid recapture. San Leo’s old cell and its story of Cagliostro now function as the endpoint of a conspiracy narrative in which identity itself can be confiscated and reassigned.

The fantasy then exposes its own machinery. Casaubon is told, in effect, that he has fallen into the plot he tried to rewrite and has used the alibi of a machine to imagine himself as a mere spectator. The remembered prison becomes a fitting image for that reversal: the man who thought he could arrange historical fragments into a romance is instead enclosed by the romance he constructed. The fortress’s significance lies in this recurrence across radically different narrative registers—first as a reported historical site that triggers an investigation, then as the imagined prison that gives Casaubon’s conspiratorial story its cruelest consequence.