for the 2006–07 season castillo agreed to join serie b side frosinone in a loan deal, but failed to impress, scoring only 5 goals in 30 matches and being mostly featured as a substitute during the campaign. for the following season he signed for newly promoted serie b club pisa and presented himself by scoring two winning goals to bari in his debut match with the "nerazzurri". at the end of the season he had scored 21 goals, gaining fifth place in the scorers table.