This area was inhabited since the period of the Falisci and in Roman times was called ‘Rossolum’ and was a stopping point along the Via Cassia. Its proximity to this great artery assisted the arrival of the conquerors and the Goths and Lombards sacked the town.
The first document that mentions it dates back to 1081, when Pope Gregory VII assigned it to the abbey of St. Athanasius. Throughout the Middle Ages it was part of the possessions of this monastery and was ruled by famous families, such as Anguillara, the Orsini and the Prefects of Vico.
In the Middle Ages also the Via Francigena passed for Monterosi, and its geographical position made it an obligatory point of passage for those who went down to Rome. In 1155 Frederick I Barbarossa, who wanted to be recognized as emperor, arrived on the shores of the lake to meet Pope Adrian IV, but his refusal to help him get off the horse prevented him from immediately becoming emperor.
In the thirteenth century Frederick II, fighting against Pope Innocent IV, built a castle on the hill, Mount Torre, overlooking the town.
Around 1400, in a period of relative calm, the population moved to the valley.
Monterosi was the scene of another great historical event in 1649 when the assassins of Farnese is murdered Bishop Monsignor Giarda who Pope Innocent X had assigned to the diocese of Castro. After this episode the pontiff razed Castro, the capital of the Farnese duchy.
Monterosi was then ruled by other families and the Village Aldobrandino was the last inhabited settlement born at the turn of the sixteenth and seventeenth century by Cardinal Ippolito Aldobrandini, Pope Clement VIII.
For centuries Monterosi life turned around “L’Antica Posta dei Cavalli”, i.e. the changing post established in 1533 for changing horses that were weary from the way. The town was a succession of inns and stables for animals.