According to legend, Montalto was founded in the 5th century A.D. by the inhabitants of a coastal city who withdrew to the small elevation where the ancient village now stands, in order to defend themselves from raids by pirates and then the incursions of the Saracens. Montalto emerges from legend and enters history in 853 A.D. following a bill by Pope Leone IV addressed to Virobono, Bishop of Tuscania. The name Montis Alti appears for the first time. The papal document states that the Montis Alti Castrum belongs to the Diocese of Tuscania, which it also served as port. The Montis Alti Castrum formed a boundary, both in relation to the Saraceninfested sea and in relation to Tuscany, which was occupied by hostile Longobards and, “also because of its immense, uncultivated, pestilential territory, soon became a valuable stronghold, a castle, a castrum”. Orsini Castle, famous monument of Montalto, also has a legendary origin. Its dates back to Desiderio (VIII century), duke of Tuscia, then king of the Longobards. For centuries, the Montalto territory was devastated by the struggle sustained by the powerful barons of Vico, feudal lords of Montalto since the middle of the 12th century, against the Pontiffs and the States allied to them. This long struggle between the Popes and the barons of Vico almost annihilated the Montalto Castle: the countryside was in ruins, the houses abandoned, and the population decreased fearfully. To prevent Montalto being erased from the land and from history, Pope Martino V published a bill on 28 February 1421 in favour of the few Montaltesi remaining in the region, and also took an interest in the state of the land, the ruined houses and the abandoned vines. After the defeat of the powerful lords of Vico in 1359, Montalto Castle started to pass from hand to hand. It was dominated by the Orsini, by Angelo Ventura known as Tartaglia, by other lords and, naturally, by the Popes. With the constitution of the Duchy of Castro, in 1537, decreed by Pope Paolo III Farnese in favour of his son Pier Luigi, Montalto (and the other towns included in the duchy) enjoyed a period of tranquillity. With the destruction of Castro in 1649, decreed by Pope Innocenzo X, the Duchy of Castro was re-confiscated amongst the goods of the Church. The unit of land was split up and granted as a concession to various lords. In 1870, with the taking of Rome, the dominion of the Popes ended. Montalto entered the unitary state.