Castellazzi followed Brescia promoted to Serie A in the summer of 2000. He made his Serie A debut on 21 January 2001, against Vicenza. In the 2001–02 season, he was first choice for the team but during the next season lost his place to Matteo Sereni. After being loaned to Reggina and Catania in the 2002–03 season, he finally became a regular starter again in the 2004–05 season, in which he played all 38 Serie A matches. Castellazzi initially the first choice of 2003–04 season, but lost his place to Federico Agliardi after matchday 4 and regained his place on matchday 23.