He was OK. Had an iron chin, was deceptively crafty, and could hit pretty good; but on the flipside, was woefully unskilled and limited. Famously pulled out a fight with the then unbeaten John David Jackson when he was literally a split second away from being stopped himself; that's probably his defining moment. His limitations were exposed by Takehara and Terry Norris, and he escaped with some questionable decisions over Jackson (in their rematch), Reggie Johnson, and others.
He's was a good fighter at middle and super middle. Had 2 wins over Jackson (gave Jackson his first loss) and 2 wins over Reggie Johnson which all fights were close He has a very solid chin, Roy Jones gave him everything he had and couldn't knock him out. Very tough fighter and pretty nice power.
What's questionable ? he had him down twice in 2 separate rounds in that fight . Anything else from a decision win in such a fight would have been a robbery.
According to boxrec, Castro boxed 928 rounds in professional fights. That's some old school type activity there.
You'd think that after getting off his stool nearly a thousand times, he might sharpen up his technique a bit.