in Città di Castello in Umbria. The family had moved to Rome in the Middle Ages. She was the goddaughter of Filippo I Colonna, her husband's employer. Giulio was the eldest of six children, two boys and four girls.

According to unverifiable anecdotes, Giulio was admitted at the age of seven to the Jesuit Collegio Romano, which had acquired an international reputation for excellence. Professors included the German mathematician