He ended his career with Napoli (1978–85), after seven seasons with the club, collecting 203 appearances in Serie A.

[2][3] He earned the only international cap in his career on 26 January 1977, coming on as a substitute in a 2–1 win over Belgium.

He is currently a member of the Italy U21 staff, and he has also worked with the Inter youth side as a goalkeeping coach, and as a scout, since 1989.

[5] Castellini was an agile, courageous, flexible, and acrobatic goalkeeper, who possessed excellent reflexes, and who excelled in the air due to his athletic ability; he was also known for his speed when rushing off his line, as well as his ability to come out and punch the ball out of the area.

During his time at Torino, he was given the nickname "The Jaguar" by the fans, due to his diving and shot-stopping ability.