Around the thirteenth century on the highest part of a hill, Nespolo was formed by the union of some villages which had previously been isolated.
The first news of this place is from 1024 when the abbot of the Abbey of Farfa has rented to Crescenzio of Rattone certain assets held in “aquam nespuli”. The village dates from the thirteenth century and is formed for defensive purposes on a hill where they would meet the people from the three nearby villages.
At Nespolo passed the border between the Papal State and the Bourbon Kingdom.
It is assumed that, before the castle, there was a convent building dedicated to St. Sebastian, the patron saint against plague and epidemics.
Until 19 December 1817, i.e. when it was released by Prince Francesco Barberini who renounced his rights on his barony, Nespolo was part of the Barony of Collalto; in 1853 it seems that there were established a shop of general goods, another of leather and that of a grain handler, among the commercial and business activities most important to the town.