It was the ancient Cluviae, a city of the Caraceni: the territory was most probably conquered by Lombards in the 6th century. The medieval name “castri de Casule” was first recorded in 878 AD in the Memoriatorium abbatis Berthari, a manuscript conserved in the Abbey of Monte Cassino.
The village was controlled, in the fourteenth century by the Orsini family, who fortified the Norman castle. Casoli became famous as part cultirale in the nineteenth century, when the Mayor Pasquale Masciantonio housed in the castle the poet Gabriele d’Annunzio. In World War II, Casoli not suffered severe bombing, because it was declared a “free city” for the displaced.