His illegitimacy tormented Pier Luigi all his life, and doubtless contributed to the formation of his character. The nobility of Piacenza was frequently known to insult him as "the bastard son of the Pope." As the eldest and beloved son he was legitimised along with his brother Paul at the age of two in 1505 by Pope Julius II. He was given a famous humanist tutor, Baldassarre Malosso di Casalmaggiore, nicknamed "Tranquillus," and quickly developed a love of war and fortifications.