|Date of birth||12-12-1945|
|Seasons in Monza||1965-1970|
|Caps and goals||60 (-43)|
He grew up in the ranks of Monza where he played in Serie B and Serie C. In 1970 he landed in Turin and was immediately promoted to owner. With the grenades he opens a cycle that will last eight seasons by collecting 264 appearances (201 in Serie A, 42 in the Italian Cup and 21 in Europe) by winning a championship and an Italian cup. In 1978 Turin, after having bought the previous year Giuliano Terraneo, sold Castellini to Naples. He played seven seasons with the Neapolitans, with 202 championship tokens to his credit, retiring in 1985. In the past years in Naples he won the Guerin Sportivo ranking as the best goalkeeper of the championship every year, and once that of the best player ever.
In the national team, closed by Dino Zoff, he has only one presence in Italy-Belgium on 26 January 1977 (2-1 the final result in favor of the Azzurri), a game in which he played the second half, conceding a goal on a penalty kick by the opposing goalkeeper, Christian Piot. He also has 4 appearances for the National B debuting on February 17, 1971.
After retiring from competitive activity, in 1989  he was hired by Inter as goalkeeper coach. He also took on the role of head coach, replacing Roy Hodgson for two games in 1996-1997 and 1998-1999 for four games as a substitute for Mircea Lucescu, before Hodgson's return. He also collaborates with the Italian Under-21 staff.