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 the seventeenth century Costanza Sforza transformed the military structure into a luxury residence, with frescoes, bas-reliefs and oversaw the creation of a park.