On the mountain top above Villa San Michele are the ruins of a fortress. They consist of a very highly located central section, in which Munthe built a simple sleeping accommodation called Villino Barbarossa, and the surrounding walls.
As the defensive fortification of Anacapri, the fortress dates from about the year 1000 and has undergone a series of reconstructions over the centuries. The historical name of Barbarossa refers to the corsair Khair-ed-Din, who in the 16th century captured all of Capri several times.
Munthe also acquired the mountainside in order to create a sanctuary for migratory birds. The area is still a natural oasis today, watched over by the Capri Bird Observatory, which is located in the habitable part of the ruins.