Here's a report from BA...
"To call Castro a late bloomer would be an understatement. The Tigers signed him out of Venezuela as an amateur in 2011, but after two years in the Venezuelan Summer League, a season lost to Tommy John surgery and a repeat of the Gulf Coast League, he didn’t make his full-season debut until 2017 as a 22-year-old at West Michigan. He has looked better at each stop since then, including a standout 2019 campaign as a starter and reliever in Erie, where he had some of the best power pitching on a staff that included top prospects Casey Mize and Matt Manning. A strong Arizona Fall League stint caught scouts’ attention, leading the Tigers to not only re-sign him but add him to their 40-man roster. But after making his big league debut out of the Tigers’ bullpen in 2020, Castro was claimed off waivers in December by Toronto.
The intrigue around Castro centers around a fastball that sits at 93-95 but works up to 97 mph with cutting action. His slider was a quality chase pitch for him against Double-A hitters, while his changeup has shown enough improvement to throw in different counts. The combination has enough movement that hitters struggle to square him up, evidenced by a low home run rate. However, it also costs him in command, coupling a 10.2 K/9 rate with 5.7 BB/9 in 2019.
Though most scouts view Castro’s best fit as a reliever, he actually has been much stronger in a starting role, where he’s better able to repeat his upright delivery. After claiming him off waivers, the Blue Jays may explore developing Castro as a starter, knowing that he could always fill a long-relief role if that doesn’t work out."
I've been preaching on this dude, he's got filth in his arsenal.