The Castello dei Principi d’Acaja is the symbol of Fossano. Arriving on the outskirts of the town, the first thing visitors notice is its four powerful square towers, arranged along the diagonals and connected by crenelated enceintes. 
Built as a defensive structure by Philip I of Acaja between 1324 and 1332, it was then transformed by the House of Savoy into a noble residence. In the sixteenth century it hosted Bona di Savoia, Duchess of Milan, and in the forties of the following century, Madama Cristina of France. 
Emanuele Filiberto also stayed here with his wife Margherita di Valois. In November 1562, in the presence of the cardinal of Lorraine, the bishop of Orleans and the lords of Alluye and Birague, Emanuele Filiberto signed the Treaty of Fossano in the castle, ending a long period of hostility between the Savoy and the French.

At the end of the seventeenth century the function of the Castello changed, becoming a prison for the Waldensians between 1686 and 1687, then barracks and prison. The town library is now housed in the sixteenth-century arcades, with over 150,000 volumes and a historical collection - consisting of ancient books, series and documents - of approximately 10,000 texts. The inner courtyard and the hall on the ground floor host cultural activities, conferences, exhibitions and concerts.

The tour itinerary proposed by the city Tourist Office, located inside the courtyard of the manor, includes the Sala delle Grottesche, created by Flemish painter Giovanni Caracca, the only decorated room to survive until now. The amazing view of Monviso and the entire Alpine arc of southern Piedmont from the panoramic tower is incomparable, where a special multimedia set-up offers visitors the opportunity to go on a journey to discover the Cuneo area.