 Four years later he rejoined his cousin Gian Giacomo Medici to fight against the Protestant coalition in Germany; he was now a general in the artillery corps fighting against the Duke of Saxony.In 1551, still with 300 men on foot, Serbelloni was sent by Ferrante Gonzaga to defend Asti against the French; he conquered the town of Saluzzo and was appointed as its governor.Always with his fierce cousin, he commanded the Florentine artillery in the war for Siena in 1554; the next summer he assaulted and overcame Porto Ercole fortress after a four-day artillery barrage, then with 700 German Landsknechts he defended Populonia against a Turkish sea-borne assault, receiving support from the Florentine cavalry.