Castle of Montessoro, Medieval castle in Isola del Cantone, Italy.

The Castle of Montessoro is a stone fortification perched on elevated ground in Isola del Cantone with thick defensive walls and towers built using traditional Ligurian construction methods. The structure displays robust architectural design intended to withstand external attacks.

The fortress was built during the medieval period and remained connected to the powerful Spinola family, who controlled extensive territories in the region. It served as a key strongpoint for monitoring and defending these lands.

The castle gave its name to the Marchesato di Montessoro, the small Spinola fief of the Luccoli branch that was enrolled among the Imperial Fiefs. Until 1797 this autonomous statelet, under Savoy protection, also counted Piano, Arquata, Serravalle Scrivia, Busalla and Voltaggio among its lands.

The ruins stand on the Bric Castellazzo above the Montessoro hamlet of Isola del Cantone. The site is reached on foot over uneven ground, so dry weather is preferable. Consolidation and recovery work has been under way since 2014, and some areas may be fenced or obstructed.

Local histories place a first building on Bric Castellazzo as early as the 11th century, under the jurisdiction of the Diocese of Tortona; some ruins still lie on that summit, separate from the 14th-century castle whose remains stand lower down.