See Porto Ercole guide for highlights and historic monuments
Roman town of Portus Herculis and Etruscan Cossa
The history of Porto Ercole in Tuscany [Latin name "Portus Herculis"], is closely linked to that of "Cosa", or "Cossa", which the Greeks called "kossai." While the older settlement of Cossa may be of Etruscan origins , "Portus Herculis" does not go back beyond the second century BC. and is actually evidenced only by later sources, such as in Rutilius Namatianus (5th century) [“De Reditu suo”, I, V].
We know from documents and inscriptions that, in Roman times, the region belonged to the powerful family of the Enobarbus. In an inscription [CIL, XI, 2638], we read:
“L. DOMITIVS / AHENOB / L. / PAPVS Domitia / Papei / L. Arche” The inscription was found “in loco qui dicitur Cerreto, 3 m.p. ab Orbetello” [translation: "in the place called Cerreto, three miles from Orbetello”] .
Some scholars have identified “Lucius Domitius Ahenobarbus” (consul 54 BC) with the proud opponent of Julius Caesar (100-44 BC). The harbour which now belongs to “Orbetello”, between the promontory and the coast:
“from the first century AD, was called ‘Porto Ercole’, in honour of Hercules” 
Connecting Porto Ercole with ancient Cossa
According to some historians, there even exists the possibility that, in ancient times, the Port of Cossa and Porto Ercole were the same place. The hypothesis stems from the fact that some ancient geographers mentioned alternatively the ‘Port of Cosa’ and ‘Portus Herculis’. Rutilius Namatianus, after describing the walls of the abandoned Cosa, added:
"Haud Procul hinc petitur signatus ab Hercule Portus" [ We focus on the harbour which takes its name from Portus Herculis, which is not far from here ] .
Portus Herculis is also cited on the Peutinger Table, as well as in the “Itinerarium per maritima loca” [journey through sea places], which does not even mention Cosa. On the contrary, Ptolemy and the ‘Itinerary of Antoninus’ cited only Cosa. Finally, the ‘Anonymus Ravenna Geographer’ [first half of the 9th century] and Guido the Geographer (died 1169) mentioned, in place of Porto Ercole , the 'Portus Cosanus'.
The place of Portus Herculis is the modern “Porto Ercole”, and it’s unlikely that the “Portus Cosanus” was separated from the Portus Herculis (D. Anziani). D. Anziani wondered:
“Why [would there have been] two adjacent ports? [...] we do not discover at the foot of the hill of Cosa any place that deserves the name "Portus." Desjardins rightly underlines that, if there had been two separate ports the “Itinerarium per maritima loca”, highly detailed about the entire Tuscan coast, would discuss this matter” .
In conclusion, according to D. Anziani, in the Ancient World there would only have existed the Portus Cosanus, and “Cosa”, which, incidentally, was one of the most famous ancient cities of Tuscany.
What was Cosa?
The Roman colony of Cosa was founded in 273 BC, after the defeat of the Etruscan city of Vulci. Already lapsed by the first century BC, this depopulated city survived until the 3rd century AD as a modest service town in favour of a country which was still thriving. However, the most recent historiography generally tends to keep the two ports separate, with "Portus Herculis" put into operation by the second century BC:
"In the second century BC, Portus Herculis could be put into operation, as evidenced by submarine explorations" .
Porto Ercole in the Middle Ages
The first mention of “Porto Ercole” in the Middle Ages dates from 805 AD, in a donation of Charlemagne (742-814) to the Monastery of “Tre Fontane”, which received, among other goods, "Portum Erculis."However, as has been amply demonstrated by scholars, this document attributed to Charlemagne is a fake made by the monks of the Monastery to ennoble it, tying it to the name of one of the most powerful Medieval kings.
Nevertheless, there is evidence for the existence of an imperial donation dating, if not to Charlemagne, to Carolingian age at least, so despite the false document, we know of the site of "Porto Ercole" in the Middle Ages, as well as in Roman times.
In the Middle Ages Porto Ercole was the subject of a dispute between the Abbey of the “Tre Fontane” and the powerful family of the Counts Aldobrandeschi, lords of vast estates in Maremma. In 1300 the town came under the rule of the Orsini Counts, and finally under the rule of the King of Naples and the city of Siena.
After the conflict between the Sienese and the Spaniards, the town was included in the so-called "Stato dei Presìdi." In this long period of Spanish domination, the territory around "Portus Herculis" was even more strengthened with the creation of other forts to protect the port, still the primary reason that tourists visit.
In the 18th century Porto Ercole was ruled by the Austrians and the French, who included it in the "Kingdom of Etruria." Later, the town passed to the Grand Duchy of Tuscany and, finally, to the Kingdom of Italy in 1861.
References (for above)
1. “Cusi” or “Cusia”, according to M. Pallottino ("Nomi etruschi di citta", Scritti in onore di B. Nogara, Roma, 1937, pp. 341-358)
2. See “Corpus Inscriptionum Latinarum”[ Inscriptiones Latinae Antiquissimae], n. 1344, edidit Theodorus Mommsen, Berolini, 1863, Vol. I, p. 257
3. See A. Solari, “Topografia storica dell’Etruria”, Pisa, Spoerri, 1920, p. 154
4. See D. Anziani “Mélanges d'archéologie et d'histoire, Année 1910, Volume 30, Numéro 30, pp. 373-395
5. See M. Celuzza, “L’epoca romana”, in “Paesaggi d’Etruria”, Roma, 2002, p. 137
References for the Porto Ercole guide
1. “Porto Ercole: il restauro di Forte Filippo”, in “Prospettiva”, 1976, p. 76 ff.
2. C. A. Nicolosi, “Il litorale Maremmano, Grosseto-Orbetello”, 1910, p. 161
3. G. Caciagli, “Lo Stato dei Presìdi”, in “L’Universo”, 1970, p. 1066
4. AA.VV., Porto Ercole. L'ultima dimora di Caravaggio”, Roma, Armando, 2010, pp.. 49 ff.
The above references are for our Porto Ercole travel guide