The origins of Castrocielo date back to the sixth century AD when, after the destruction by the Lombards, groups of Aquino families took refuge in a castle on the Asprano mountain. For the position in which they found themselves, the castle was called “Castrum Coeli” (Fortress in the sky). In 994, when the twenty-eighth abbot of Montecassino went up the mountain he found only abandoned ruins as a result of the lack of water and started to build a new fortress. For this reason the foundation of Castrocielo date is set to 996.
When the period of the raids ended, the population began to descend to the valley and founded the towns of Colle San Magno and Castrocielo.
After the Abbey, Castrocielo has been ruled by the families of D’Aquino, then in 1583 the Boncompagni and after 1796 was part of the Bourbon kingdom. Because of its location on the border, after the unification of Italy it was enlivened by bandits. Castrocielo was bombed heavily during the Second World War.