In the early Middle Ages the town was, alternatively, under the rule of the Byzantines and the Lombards, until it became part of the county of Sora in the Principality of Capua. Isola del Liri was the feud of the dell'Isola (de Insulae) family from the 12th century to the 14th century. Members of the family include Roffredo dell'Isola, Abbot of Monte Cassino, Count d'Arce, and Bartholomew dell'Isola who enjoyed the honors in the seat of Capuano Naples. At the end of the 14th century, Isola del Liri was ceded to the Cantelmo family.