Batting in front of slugging prospects Jonathan Singleton (Astros) and Javier Baez (Cubs), Castellanos hit the go-ahead homer as the Mesa Solar Sox beat the Surprise Saguaros, 6-5, on the third day of the Arizona Fall League season.
With the game tied at five with one out in the ninth inning, Castellanos took Edwin Carl (Royals) deep on the fifth pitch of the at-bat, giving the Solar Sox their first lead of the game.
"I just stayed confident," he said. "I didn't start well, I started 0-for-3. I had [the mindset] that I needed to cover the outside of the plate and not give that up. Secondly, I was just trying to battle and somehow get on base because it was a tie game. With two strikes, I battled and he hung a curveball."
Batting third in the Solar Sox lineup, Castellanos benefits from having Astros' top prospect Jonathan Singleton in the cleanup spot and Cubs' top prospect Javier Baez batting fifth, no slouches when it comes to swinging the lumber.
"Having them behind me is huge," said the Tigers' top-ranked prospect, who also singled in five at-bats. "I didn't have that this season."
After falling behind 5-1 following the third inning, Mesa put up single runs in the fourth, fifth and sixth frames before tying the game in the eight. Entering the game winless through the first two games of the fall campaign, Castellanos' second hit of the night was enough to give them the edge.
"The first one [win] is always the hardest to get," the 20-year-old outfielder said.
The Florida native has gotten off to a strong start, collecting two hits in each of his first three games. He's scored three runs and driven in two for the Solar Sox.
Castellanos split the 2012 season between Class A Advanced Lakeland and Double-A Erie. After batting .405 in 55 Florida State League games, Castellanos saw his numbers come back down against more advanced pitching, posting a .678 OPS in the Eastern League. Overall, the Florida State League All-Star batted .320 with 10 homers and 57 RBIs in 134 Minor League contests.
"Really good, but by no means am I where I want to be," Castellanos said of his year, "so there is a lot of work to do."
Castellanos said his main focus during the Arizona Fall League will be improving his defense in the outfield, specifically, getting better reads off the bat from his right-field spot.
"In the fall league is the best I have felt [defensively]," the converted third baseman said. "My focus is much better."
Singleton and Baez each contributed two hits, with Singleton scoring twice and Baez driving in a run.
Saguaros pitcher Seth Blair began the game with three one-hit innings, allowing one unearned run on a wild pitch. The Cardinals' 2010 first-round Draft pick struck out five batters and walked two others.
Solar Sox starter Dae-Eun Rhee (Cubs) gave up two runs on six hits over two innings. Astros' 11th-ranked prospect Nick Tropeano struck out three over two perfect innings of relief but did not factor into the decision.