The ancient castle of Capaccio was built in 1246 on Mount Calpazio, in what today is called Capaccio Vecchio. Built to protect the city from increasingly insidious Saracen invasions in 1246 - under the dynasty of Svevo - the castle became the protagonist of what will go down in history as the Conspiracy of Capaccio, a resolute revolutionary action that saw the feudal lords Campanian ally alongside the Pope Innocent IV, countering the power of Frederick II of Swabia.
The conspirators, attacked by the Emperor's troops, took refuge in the castle of Capaccio Vecchio, resisting for about three months until the final conquest. The conspirators were then killed and Capaccio Vecchio underwent a slow, inexorable depopulation. The few survivors found refuge in the village of San Pietro (situated on the slopes of Mount Sottano in the historic Monticello area).
The castle was later renovated under the Angevin dynasty and used as a prison. Towards the end of 1400, openings were created in the stone structures for the use of cannons but castle remained unused until the end of 1800, when it was utilized by the military for telegraph lenses.