Mesocco Castle, perched high on a rocky outcropping dominating the Moesa Valley and likewise forming the barrier between the subalpine and the alpine regions of the valley, was the main fortress of the Grisons and one of the largest fortress complexes of Switzerland. The original medieval fortification was enlarged in several phases by the von Sax family, feudal lords of the valley (11th-14th Centuries) and after 1480 it was transformed into a renaissance fortress, replete with guns, by Gian Giacomo Trivulzio, lord of the Moesa Valley. Upon orders of the Rhætian Leagues, the fortress was dismantled in 1526 for strategic reasons (to prevent a foreign power from taking control). From 1925-1926 and at the end of the 20th Century the ruins were consolidated and restored. The fortified outer wall of the castle comprises five prominent towers.
Access to the inner court, an irregular, five-sided plan, is through the North East tower. Inside, one may observe the remains of residential and service quarters (storage rooms, stalls, foundries, ovens, etc.). Also in good condition are the ruins of the church of San Carpoforo, a Romanesque construction from the XI Century, complete with a harmonious bell tower dendrochronologically dated to 1066-1067. The principal tower of the complex, of which an imposing square base remains, was destroyed during the 19th Century, probably due to a lightning strike. At the base of the castle, archeological excavations have uncovered, among other finds, evidence of a human presence during the Mesolithic period (ca. 9000 B.C.) as well as ruins of a barrier from the late Roman period.