The castle ruin of Castello Mesocco sits on a rock outcrop in the Italian-speaking southern Mesolcina Valley (Misox).

It is one of the largest castle fortifications in Switzerland, and from the beginning of the 13th century to 1526 it was the lordly center of the valley. Considered impregnable, the castle was never conquered. Below the castle is the church Santa Maria de Castello, adorned with a well-preserved Late Romanesque fresco cycle.