[3] Casoli borders the following municipalities: Altino, Civitella Messer Raimondo, Gessopalena, Guardiagrele, Palombaro, Roccascalegna, Sant'Eusanio del Sangro.

[4] It was the ancient settlement of Cluviae, a city of the Caraceni tribe that was the territory 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.

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 did not suffer severe bombing, because it was declared a "free city" for the displaced.