Castro went 3-for-5 with two doubles and a career-high six RBIs in the Double-A Reading Phillies' 19-6 thumping of the Richmond Flying Squirrels.
"He has exceeded our expectations," R-Phils manager Dusty Wathan said. "He missed the second half of last season with a stress fracture in his shin after having a great first half. It would have been a slam dunk that he would have been in Reading at some point last year if he was healthy."
Signed by the Phillies as a non-drafted free agent in 2007, the 21-year-old is now batting .333 with 14 RBIs in 11 Eastern League games.
The team's No. 15 prospect smacked a two-run double to left field in the first inning, and he added an RBI single in the fifth. A sacrifice fly in the top of the sixth made it 11-1, and he smacked a bases-loaded two-bagger later in the inning as the Phillies batted around and sent 15 men to the plate.
He had a chance to expand on his RBI haul when he stepped to the plate with two men on base in the eighth, but grounded into an inning-ending double play.
"He was comfortable in the box and he was seeing the ball well," added Wathan. "I think it's a situation where he's still learning the strike zone. He's a good fastball hitter and he's been laying off the breaking balls.
"He has a little bit of a free swing, but he is very aggressive. He came through in key situations with runners on second and third, and he had four two-out RBIs, so he was clutch tonight."
Castro bested his previous RBI high by one. He had plated five runs twice before, most recently in Class A Lakewood's 12-6 win over Greenville on May 14, 2010.
But Castro was not the only Phillies prospect to feast on Richmond pitching Tuesday.
Cesar Hernandez, the Phillies' No. 14 prospect, went 5-for-6 with five runs scored out of the No. 2 spot, while first baseman Darin Ruf missed hitting for the cycle by a triple as part of a 5-for-5 performance.
Wathan said: "It was a great team win. We were 12-for-12 with runners on third base and less than two outs, so our situational hitting was good up and down the lineup.
"It's obviously a lot of fun. We're swinging the bat well and that makes everyone comfortable. Scoring runs puts our pitchers at ease and gives everyone a lot of confidence."
Reading starter Trevor May, the team's top prospect who is also ranked No. 53 on MLB.com's Top 100 list, struck out seven batters over five one-run innings to improve to 3-0. The fourth-rounder scattered four hits and walked two batters and lowered his ERA to 3.18.
"Trevor didn't have his best night, but he was able to get through five innings," Wathan said. "He didn't have his fastball, but his changeup was really good and he got the key outs when he needed them."
Richmond's Jason Stevenson surrendered eight runs on 10 hits and two walks while striking out three batters over four innings in the loss.