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, Lazio, 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.