OK, I never heard of it either… What is below is lifted from Wikipedia. I know nothing about Presidi, but as a fringe territory near a number of interesting places, I wonder if it may have played a stepping-stone role, perhaps as Bidache by France may have done. This is total speculation, but may be worth future investigation. It was a Spanish puppet, midway between Rome and Livorno. Spanish power was concentrated in the south of Italy.
“The Stato dei Presidi (loosely in English, “The State of the Garrisons”) was a client state of the Kingdom of Spain situated in central Italy, which included the cities of Orbetello, Porto Ercole, Porto Santo Stefano, Talamone, Ansedonia and Porto Longone, in what is now southern Tuscany. Porto Longone included a fortified area of Elba Island.
The State of Presidi was created by King Philip II of Spain in 1557 out of territories once belonging to the Republic of Siena, after the latter had been annexed to the Grand Duchy of Tuscany by Cosimo I de’ Medici.
The State of Presidi was governed directly by Spain, as part of the Kingdom of Naples. It was merged to the land of Ferdinand IV of Naples with the duchy of Sora in 1796. It followed the history of the latter until, in 1801, Napoleon disbanded it and annexed it to the newly formed Kingdom of Etruria. Later the Congress of Vienna (1815) gave its former territories to Tuscany, to which they belonged until the Unification of Italy.”