The patron saint of Cosseria is Saint Bartholomew, celebrated on 24 August. The parish church of the Immaculate Conception, in the main village, was rebuilt between 1656 and 1660 on an earlier building and restored in 1831: it holds a fine rose window on its façade, 16th-century frescoes in the sacristy, and a pipe organ built by the Bergamo craftsman Pietro III Perolini between 1887 and 1888.
The town, however, is best known for the bloody battle fought on 13 April 1796, when Napoleon Bonaparte, in the so-called battle of Millesimo, defeated the Austro-Sardinian army troops who defended the castle with great valour, losing their lives along with their commander Filippo Del Carretto: more than three thousand soldiers died in the clash, and just as many were wounded, opening with the French victory the occupation and subsequent annexation to France of the mountain territories of Liguria and lower Piedmont. The castle, amid whose ruins the battle took place, had already been demolished in 1553 by the imperial commissioner Gerolamo Sacco di Ceva, by order of governor Ferrante I Gonzaga. The 17th-century church of San Rocco housed the command of the Napoleonic troops during the crucial phases of the fighting in the Bormida valley; in 2006, for the bicentenary of the battle, the territory hosted an extraordinary re-enactment of the event with over five hundred participants.
