The authors agree that this surname derives from castello and its toponymic variants distributed throughout Italy. In most cases it represents gentilic names and nicknames linked to place names formed or derived from castello, such as Castelletto, Castellini, Castelluccio, Castiglione, Castellaro, Castelfranco, Castelnuovo and Castelvecchio, among others. In some cases it can continue a denomination —nickname or trade name— given to the one who lived or worked in a castle.

E. De Felice notes that the surname is frequent throughout Italy, with specific regional distributions: Castiello is peculiar to the Naples area, Castellaneta to Puglia, Castioni to Lombardy, and Castellari predominates in northern Italy. He also indicates that Castiglio represents a loan from the Spanish surname Castillo, of the same etymology.

F. Miranda groups this surname within the same family that includes Castellano, Castellani, Castellaro, Castellazzi, Castello and Castelluccio, all probably derived from nicknames related to residence or dependence in a castle, or from place names with the root castello.

For its part, it is considered that the localities of origin are situated mainly in the Bergamo and Brescia area, although the exact provenance of the surname is not entirely clear.