Yo Kass answered
The name Castellano is fairly widespread along the Italian peninsula, with regions boasting a large number of fellow Castellani ranging fom Piemonte in the north, to the island of Sicily in the south.
However, despite its popularity in Italy, the name Castellano can by no means be exclusively attributed to an Italian heritage. The name itself derives from the Latin word for 'castle' which is Castellum and is corroborated by the various Castellano coats-of-arms that bear the symbol of a turret or castle.
Despite this information, it would be near impossible to geographically pinpoint lineage, especially considering the first evidence of forts and castles can be traced back to the 9th century BC and have appeared all over Europe since.
To highlight this point, the name Castile refers to a historical region of Spain that gives its name to Castellano - a term which is represents the 'standard' Spanish (as spoken by TV and Radio presenters and journalists in Spain), a term especially used to contrast the Spanish spoken in Spain from that spoken across the Atlantic.
Therefore, although it is possible the origin of the name Castellano is Italian, it is by no means an exclusivley Italian surname.