He spent 2 seasons on loan at Lombardy side Pro Sesto, then sold to Cagliari in co-ownership deal.
He then left on loan to Serie C1 club Reggiana, where he rejoined ex-Inter teammate Raffaele Nuzzo.
With Nuzzo and Luca Mondini, Castelli worked as the 3rd choice keeper and in January 2003 left for Reggiana's league rival Prato,[3] as understudy of Alessio Sarti.
In 2004–05 season he left for Serie C1 side Spezia along with forward Nicola Napolitano, which at that time the club partially owned by Inter.
[5] He competed with Maurizio Monguzzi for the first choice position but in January 2006 left for Serie C1 and Lombardy side Pro Patria.