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. While technically a vassal state of the Papal States, it 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 di Ferentino.