thumb|262px|Castel Sant'Angelo, Rome
Pier Luigi Farnese (19 November 1503 – 10 September 1547) was the first Duke of Castro from 1537 to 1545 and the first Duke of Parma and Piacenza from 1545 to 1547. He was the illegitimate son of Cardinal Alessandro Farnese (who later became Pope Paul III). He became a soldier and participated in the Sack of Rome in 1527.
Youth
Pier Luigi Alessandro Farnese, born in 1503, was the son of Cardinal Alessandro Farnese and Silvia Ruffini. His father would be elected Pope Paul III on 13 October 1534.
In July 1505, Pope Julius II legitimated Pier Luigi so that he could inherit the Farnese family estates. On 23 June, 1513, Pope Leo X published a second legitimation of Pier Luigi. In spite of the legitimations, 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." He was given a famous humanist tutor, Baldassarre Malosso di Casalmaggiore, nicknamed "Tranquillus," and quickly developed a love of war and fortifications.
His father was, however, keen to make Pier Luigi the true head of the Farnese family and so arranged a favourable marriage alliance with Gerolama (also known by the spelling Girolama) Orsini, daughter of Lodovico, Count of Pitigliano. In 1513 the engagement contract was drawn up, and in 1519 the wedding celebrated.
Despite a loveless marriage, Gerolama remained a faithful devoted wife, tolerating Pier Luigi's excesses, brutality, and extravagances with dignity. Delays in the construction at the palace in Gradoli, meant the young couple had to lodge in the Castle at Valentano. The following year their first son Alessandro was born.
Military career
Pier Luigi Farnese quickly became the stereotype of a mercenary soldier: wild and amoral. He had courage and daring and while strong and audacious was also sufficiently brutal to offend many observers. Nor did he always fight on the traditional side of the papacy; reversing the pro-Guelph sentiments of the Farnese.
In 1520, at the age of seventeen, he and his brother Ranuccio were already employed as mercenaries in the pay of the Republic of Venice. As a result, he served under the standard of Charles V — remaining with the emperor until 1527, he was present at the Sack of Rome of that year, in which he took part.
While his brother Ranuccio withdrew to Castel Sant'Angelo to defend the Pope; Pier Luigi crossed the Tiber and quartered his men in the family palace, thus saving it from destruction. Critics accused the Farnese of backing both sides, but Pope Clement VII refused to condemn him. Finally when the plague hit the city, the imperial troops decided to withdraw.
Pier Luigi withdrew from Rome into the Roman countryside, taxing it without mercy and permitting a climate of theft and murder. Pope Clement, tired of this behaviour, eventually threatened him with excommunication, until Cardinal Alexander tried diplomatically to reconcile his son with the pope. In 1528 Pier Luigi, still under imperial pay, fought in Apulia against the French army and distinguished himself in the defence of Manfredonia.
Captain General of the Church
When his father was elevated to the papacy as Paul III in 1534, great festivities were celebrated at Valentano, after which Pier Luigi left for Rome.
Paul's first action was to make Pier Luigi's eldest son, Alessandro Farnese, a cardinal. But Charles V only reluctantly allowed the granting of titles to Pier Luigi over the city of Novara, To this he added the territory of Ronciglione. Pier Luigi was invested with the titles of "Montalto" which gave the right to export grain without paying taxes; and Paul accepted feudal rights over Canine, Gradoli, Valentano, Latera and Marta. He exchanged the city of Frascati for the fortress at Castro, and at bought Bisenzio from the diocese of Montefiascone. As a final act, Paul created a formal duchy out of the lands and bestowed it upon his son and heirs. The duchy was, however, to come under the direct control of the Holy See.
The Duchy of Castro operated as a functioning State within the Patrimony of Saint Peter.
In 1538 his son Ottavio married Margaret of Parma, an illegitimate daughter of Emperor Charles V; thus consolidating the friendship between the Farnese and the imperial family. In 1543 another son, Orazio, was sent to France and eventually married Diane de France - an illegitimate daughter of Henry II of France. Finally in 1545 his third son, Ranuccio, was created a cardinal by Paul III.
Duke of Parma and Piacenza
thumb|262px|Pier Luigi appointed to Duke of Parma and Piacenza by Paul III
Paul III then went on to make Pier Luigi Duke of Parma and Piacenza, properties that had previously been a part of the Papal States. Pier Luigi and his son, Ottavio, declared they would pay 9,000 golden ducati every year to the treasury of the Holy See, and, in exchange, they gave back the Duchies of Camerino and Nepi. Pier Luigi took possession of his new states on 23 September 1546.
Conspiracy and death
His tyrannical rule and his taxes gained him the enmities of the cities, which were used to the more sensible authority of the Popes. In addition, Charles V, Holy Roman Emperor fostered discontent with the Duke, in part to further his aim of annexing Parma and Piacenza to the Duchy of Milan. Thus the governor of Milan, Ferrante Gonzaga, learning of the aims of the emperor, launched a plot against the Farnese, supported by the marquis Anguissola and Andrea Doria. although subsequent events led to the return of the duchy to Pier-Luigi's son, Ottavio in 1551.
Following the death of his son, Pope Paul met in consistory and had Ferrante accused of the murder - declaring that Ottavio would succeed his father as the new duke and the new Gonfalonier of the Church.