The castle has a very ancient and troubled history, about 1300 years long. In the sixth century the Lombards, coming from Benevento, settled in the area and less than a century later the castle was built, of which only a few ruined ruins remain today. As was customary in medieval times, the first inhabited nucleus developed around the fortress, what is now known as Borgo San Matteo.
In the following centuries the castle hosted several times King Manfred of Swabia, who was buried there with his son after losing his life during the battle of Tagliacozzo. Later the castle passed into the hands of the Angevins and the County of Sarno came under the control of the Kingdom of Naples. In the 15th century, before passing to the Aragonese, the castle was occupied by the Orsini family, to whom we owe the construction of the Torre dell 'Orso, still visible today. In these years the castle was the scene of the famous conspiracy of the barons and the Torre dell 'Orso was decisive for the fate of the battle of Sarno.
The history of the fortress of Sarno ends in the seventeenth century: in 1647 it was destroyed during the repressive actions against the revolts triggered by Masaniello. Also for this reason today the fortress is a set of almost unrecognizable ruins, which emerge scattered here and there in the mountain vegetation, where the central body of the castle is more intact and still practicable, albeit with a lot of attention and effort.