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Trevi in Roman times
The ancient small town of Trevi (Lat. "Trebia") was the subject of extensive studies from the mid-16th century. It was built presumably in Roman times and we have the oldest mention of it in Pliny , who referred to the inhabitants of Trevi, calling them "Trebiates".
Another important source regarding the antiquity of Trevi is found in Suetonius , in relation to a sum of money allocated to "Trebiates" for the construction of a theater:
"To grant to the inhabitants of Trevi this sum for the construction of the new theater" (Tib. XXXI).
In his turn, Pliny the Younger  recalled the Trevi area as being dotted with villas, baths and temples. Other evidence dates to 333 AD, when Trevi was mentioned in "Itinera Hierosolymitana" together with Spoleto and Foligno.
Beside the literary evidences, various Roman funerary inscriptions were found in the territory of Trevi which typically cited the name of the deceased and his loved ones. Among them we can mention an example in which a father remembers his young son who died prematurely:
“Lucius Cusinius Fortunatus dedicates this plaque to his very unhappy son and he leaves in perpetual memory to the benevolent Gods the decurion Lucius Cusinius Fortunatus, son of Lucius, who lived 17 years, 22 days".
Christianity arrives in Trevi
With regard to the spread of Christianity in Trevi, G. Cappelletti stressed that
"with the spread of the Christian faith in the other cities of Umbria, Trevi also embraced it, and because it was a pretty important city, it had its own Bishop (...) The first Holy Bishop was Emiliano, who under the emperors Diocletian and Maximian obtained the purple crown of martyrdom...
... The legends tell us that he was Armenian and (…) he exercised his heroic virtues among the faithful of the church of Spoleto, until the inhabitants of Trevi, admiring his holiness, elected him their pastor, and brought him to Rome to be consecrated Bishop of their city. Jacobilli says that this event happened in 298 AD. After some time, discovered by the pagans, he was captured and subjected to the most severe trials, and the most painful punishments. Saint Emiliano converted thousands of infidels, who likewise supported the martyrdom " .
Beyond the legendary aspects, in the area of Umbria, Christianity spread along the Via Flaminia . With regard to the martyr mentioned by G. Cappelletti presumably, as he himself says, we are faced with a legend:
“That Trevi had been home to a [Christian] community is undeniable, but that it was the seat of a Diocese is just a guess (...) We must consider the fact that until the time of Gregory the Great did not know any name of the bishop of the city” .
In fact, the introduction of Bishops takes place in 176 in Spoleto, Foligno in 203, and 298 in Trevi. The Episcopal seat lasted until 1050, when it was annexed by Emperor Henry III to the Bishop of Spoleto.
'Modern' Trevi in medieval times
In 881 Trevi suffered an attack by the Saracens and in 915 and 924 it was devastated by the Hungarians. It was later conquered by the Lombards and inserted in the Duchy of Spoleto.
The Middle Ages was a period of great changes and struggles against the Umbrian towns, in particular with Foligno. Among the most significant events we mention those relating to 1198, when Pope Innocent III (1161-1216) wrote to the podestà of Foligno, the citizens of Spoleto, Assisi and Rieti about the resignation of the Duchy of Spoleto by Conrad of Swabia (1173-1196].
The Pope crowned the Emperor Otto IV (1175-1218) in Rome, who granted Trevi to Foligno (1214). In the same year Diopoldo, the Duke of Spoleto, destroyed Trevi, ensuring the possession of the territory by the municipality of Spoleto.
What is certain is that the chronicles in 1215 showed a very conflictual relationship with Foligno. Trevi in 1285 drew up a peace treaty with Foligno and then in 1288 we see its alliance with Perugia against Foligno, which was attacked again in 1308. Then in 1305 Trevi allied with Perugia, which it helped in the siege of Todi, but a short time later Trevi suffered a siege by the Perugians .
The city experienced a new period of submission to Foligno when Pope Boniface IX [1350-1404] in 1392 named Ugolino II Trinci (died 1353) as Count of Foligno, Vicar Apostolic of Trevi. With the rule of the Trinci, political and judicial power was exercised by them in the form of the Apostolic Vicariate, which, in reality, masked an undisputable domain of this family.
Pope Martin V [1368-1431] overthrew the rule of the Trinci of Foligno, and thusTrevi regained a certain political autonomy; however, it became part of the dominion of the Papal States. With a Breve of Pius VI [1717-1799] in 1784, Trevi obtained the title of city. It remained in the Papal States until the Unification of Italy in 1861.
Location of ancient Trebia
Another important issue concerned the historic location of the town. In this regard there were various assumptions, and according to some scholars it was located in an area called "Pietrarossa", while for others the town was built on the site of the current town. According to Tiberio Natalucci, the city was located:
"where now stands the church of ‘Santa Maria di Pietrarossa’, about one mile from Trevi." 
It was called "Lucana Treviensis" and it seems that it was destroyed during the Arab raids in Umbria in 840-841 AD.Therefore, the city suffered a shift after the Arab invasion. A. Bonaca  argued instead that:
"it is a pure legend that Trevi was located at Pietrarossa. Trevi was always located where it is now."
However, more recently, Pierre Fontaine, examining the previous hypothesis, says that the Roman town of Trevi was built in the area of Pietrarossa, and he observes that the shift of the town took place after the Arab invasion; so Trevi was rebuilt and fortified on higher ground for obvious reasons of defense .
Excavations in recent years have focused in particular on the ancient walls surrounding the town. Their construction dates back to four expansions, one of which dates back to Roman times; presumably here the Romans built the first fortress on the top of the hill, easily defensible, and where the city's residents took shelter in case of enemy attack; after the Arab invasion the town was permanently moved on the hill.
Contemporary studies on the city walls, that had a perimeter of 478 meters, have reached very interesting results about the time when the ancient Roman fortress was built. It was found that the walls were built between the 1st century BC and early 1st century AD, during the civil wars between Gaius Marius (157-86 BC) and Lucius Cornelius Sulla (138-78 BC), in which many fortifications were destroyed and then rebuilt or reconstructed "ex novo" in Umbria, which was not directly involved in the civil war, but frequently was a battle arena.
Beyond the archaeological discoveries relating to the founding of Trevi in Roman times, excavations discovered some old settlements dating back to the Iron and Bronze Age. With regard to the Bronze Age, it is conceivable that the settlements:
"were located along the hilly range foothills, an area that usually did not have problems with water drainage" .
Ancient origins of the name and town of Trevi
According to S. Bardetti, Trevi derives from "tref" and "Trevv", "which originally meant 'oppidum', ' fortified city' and in the case of a river means 'water'; so Trevi means 'city situated near water'" .
The hypothesis of Bardetti has been generally surpassed by contemporary studies:
"The name 'Trebia', in Osco-Umbrian = 'Trebo', means 'home', from the base 'treb / u' = build. It reveals that we are dealing with a Ligurian (Ambitrepia), Iberian (Contrebasse Lefkada) and Umbrian name. Here is a key element that linguistically helps us to connect the environment of the Sabines, in the broadest sense of the Italic people, and that of the Ligurians. We recall with G. Devoto the ‘Trebulana” gate of Gubbio. " .
Carlo Battisti develops a similar thesis, noting that:
"Trebium in Latium was studied in its relations with the Greek term 'Treboula' by F. Ribezzo in 1948 and by G. Devoto  who, making reference to Italian Apennines, notes that this toponomastic type coincides with the area occupied by the indigenous Italic peoples. However, we will combine with it ‘Treba’, the modern Trevi, which was presumably an ancient Latin commune" .
Very interesting, and perhaps correspondent to the historical truth, as it develops the idea of Carlo Battisti (Trevi=home), is the idea that the term may have a sacred value, since in the territory of Trevi the famous “Sources of the Clitumnus” are situated:
“We observe that the cult of Jupiter was rooted since ancient times in the territory in Trevi (...) Near the Sources of the Clitumnus to the southern end of the territory of Trevi is attested 'Jupiter Clitumnus', that is 'Jupiter, protector of the woods', whose worship is attested to by stones from the nearby places like San Quirico and Picciche (...) Some scholars have speculated that the toponym Trevi derives from 'Trebe', a name that was associated with the name of Jupiter, ‘Trebe Giovio’ and ‘Trebu Giovia’; they were names that are in the‘Tabulae Iguvinae’ with the meaning of ‘home’ " .
Another important cult of the Clitumnus was that of Janus, and T. Natalucci said
"there was near a clear spring a sacred temple of Janus (…) which was still visible in the 17th century, and on ruins of it was erected a Christian church, which was called "St. John of Janus."
Commenting on the passage of T.Natalucci, Gasperoni Panella explores the question of the presence of Janus, which was correlated with the conversion of Trevi to the Christian religion after the fall of the Roman Empire:
"At Trevi, in addition to the evidence of a deep-rooted worship of Janus, is attested a deep devotion to the goddess Diana. It is interesting to note that these two deities were worshipped in the same place, and often represented in pairs as symbols of the moon and sun. In fact, in historical paintings, sculptures and coins, Janus is depicted with two faces, one of which was bearded...
... the two faces were thought to be the symbol of the moon and sun, as Macrobius says (fourth century AD). In 'Saturnalia' he writes that January [Lat. "Januarius"] is dedicated to the god Janus, the god with two faces, because he was represented symbolically merging with Artemis [Lat. "Jana"], that is Diana, corresponding to 'Diana Trivia' and to 'Hecate Triform', that is the representation of the sun and moon" .
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1. Pliny [23-79 AD] (Nat. Hist., III, 114)
2. Suetonius [69 circa-175 AD] ("Life of Tiberius")
3. Pliny the Younger [61-103 AD] (Epist., VIII, 8)
4. Tiberio Natalucci (“Studi sulla storia di Trevi”, 1985, p. 21)
5. “Le memorie francescane di Trevi”, in "Studi francescani", 1926, No. 13, p. 11
6. See S. Marchi, “la forma urbana dell'antica Trevi”, in L. Quilici, “Città dell'Umbria”, Roma, 2002, p. 170).
7. See A. Maldini, “Culto e popolamento antico alle sorgenti del Clitumno”, in L. Quilici, “Città dell'Umbria”, Roma, 2002, p. 160
8. See S. Bardetti, “De' primi abitatori dell'Italia”, Modena, 1762, p. 119
9. See“Giornale italiano di filologia”, 1957, p. 36
10. G. Devoto ("Italici", p. 117)
11. See C. Battisti, “Sostrati e parastrati nell'Italia preistorica”, Le Monnier, 1959, p. 146
12. See S. Marchi, p. 172 note 34
13. See V. Gasperoni Panella-M. G. Cittadini Fulvi, “Dal mondo antico al cristianesimo sulle tracce di Giano: il simbolismo della porta e del passaggio in relazione al dio bifronte”, 2008, pp. 109-110
14. See G. Binazzi, “Inscriptiones christianae Italiae septimo saeculo antiquiores”, Edipuglia, 1989, p. XVIII
15. See G. Cappelletti, “Le chiese d'Italia ...”, 1846, p. 393
16. See “Umbria cristiana: dalla diffusione del culto al culto dei santi”,2001, p. 136
17. With regard to these convulsed historical events, See AA.VV. "Fragmenta fulginatis historiae", Zanichelli, 1933, pp . 54-60 and “Atti del Congresso internazionale di studi sull’ a̓lto Medioevo” , 2001, Vol. II, p. 848