Duchy of Castro
The Duchy of Castro was a fiefdom in central Italy formed in 1537 from a small strip of land on what is now Lazio's border with Tuscany, centred on Castro, a fortified city on a tufa cliff overlooking the Fiora River which was its capital and ducal residence. Technically a vassal state to the Papal States, it in fact enjoyed "de facto" independence under the rule of the House of Farnese until 1649, when it was subsumed back into the Papal States and administered by the House of Stampa di Ferentino. The title of Duke of  Castro has been held since the late 1860s by the claimant to the headship of the House of Bourbon-Two Sicilies, since the Kingdom of the Two Sicilies was annexed to the newborn Kingdom of Italy. Prince Carlo, Duke of Castro currently holds the tile. It was created a duchy by Pope Paul III (1534–1549) in the bull "Videlicet immeriti" on 31 October 1537, with his son Pier Luigi Farnese and his firstborn male heirs as its dukes. It only lasted little more than 110 years and was eclipsed by the Farnese's possessions in Parma. It stretched from the  Tyrrhenian Sea to the Lago di Bolsena, in the strip of land bounded by the river Marta and the river Fiora, stretching back to the Olpeta stream and the lago di Mezzano, from which the Olpeta flows. The duchy of Latera and county of Ronciglione were annexed to it.