At nineteen he became orphaned and subsequently served as a condottiero under Philip IV of France in Flanders, then with the Visconti in Lombardy, and in 1313 under the Ghibelline chief, Uguccione della Faggiuola, lord of Pisa, in central Italy.

[1] He assisted Uguccione in many enterprises, including the capture of Lucca (1314) and the Battle of Montecatini (1315), in which he was the main protagonist of the victory over the Guelph League led by the Florentines.

An insurrection of the Lucchesi having led to the expulsion of Uguccione and his party, Castruccio regained his freedom and his position, and the Ghibelline triumph was presently assured.

[1] Elected lord (as lifelong consul) of Lucca on 12 June 1316, he warred incessantly against the Florentines, though at home he renovated the Ponte della Maddalena, spanning the River Serchio.

In 1325 he defeated the Florentines at the Battle of Altopascio, and was appointed by the emperor duke of Lucca, Pistoia, Volterra and Luni; two years later he captured Pisa, of which he was made imperial vicar.