The ducal castle of Isola del Liri, or Boncompagni – Viscogliosi Castle is a fortified palace in the historical centre just where the Liri River separates into two parts. It is a national monument.

The fortified centre is of high-medieval origin and was built incorporating an original to a defence tower on the Sora plain. The first mention of this castle dates back to 1100, in a papal bull in which is mentioned a “Castellum insulae”.

With the Duke Della Rovere, the castle became the ducal residence and headquarters of the political activities of the Sora lords in the Liri valley. In 1579 Pope Gregory XIII bought the castle for 100,000 ducats for his son Ugo Boncompagni, first born to become a prelate, commander of the papal army. At the death of his father, the pope, Ugo retired to Isola del Liri with his wife Costanza.

In the 17th century, Constantia Sforza changed the military structure into a noble residence with frescoes and bas-reliefs and transformed it into a cultural centre. Constance also cared for the realization of a park and planted over 27,000 olive trees.

The castle is private and open to ceremonies and, in its grounds, is the Church of Our Lady of Graces and a still working felt manufacturing plant which exploits a hydroelectric power plant in the secondary waterfall.