Jackson Martinez struck in the 57th minute of their last-16 first-leg clash at the Dragao to hand the hosts a narrow lead ahead of the return fixture at the Stadio San Paolo.
Castro was understandably happy to see his side deny Napoli an away goal but he was less impressed with a decision that saw Carlos Eduardo's 19th-minute header ruled out for offside.
"We've had access to the images and the response is natural. The goal is clear. We should have won 2-0," Castro told Sport TV.
"Our aim was to win without conceding a goal and it was achieved. We won well. Victory suits us well, but I get the feeling that we could have won 2-0."