A year later Castelli started writing scripts for several Italian comic books, including Pedrito el Drito and Piccola Eva published by Universo, Cucciolo and Tiramolla for Edizioni Alpe, and Topolino for Mondadori.

For this magazine he created L'Ombra, a personal take on The Invisible Man drawn by Ferdinando Tacconi; Gli Aristocratici, a group of gentlemen thieves, again with art by Tacconi; Otto Kruntz, a mad scientist drawn by Daniele Faragazzi; and L'Omino Bufo, an absurdist humor strip that Castelli drew himself.

In the same year Castelli began his cooperation with publisher Sergio Bonelli, writing stories for Zagor and Mister No.

The series, initially drawn by Giancarlo Alessandrini, marked a turning point in Italian popular comics history, introducing modern and sophisticated themes in a market dominated by traditional adventures aimed at a younger audience.

Castelli also wrote a book on the first 25 years of American newspaper comics (1895–1919), entitled Eccoci ancora qui.