The Cubs’ 21-year-old prized prospect also singled to back rookie Kyle Hendricks.
Baez homered in his major league debut Tuesday, delivering a game-deciding shot in the 12th inning at Coors Field.
Baez led off the sixth inning with a shot down the left-field line against starter Yohan Flande. Baez hit a two-run shot off reliever Juan Nicasio into the Colorado bullpen in right in the eighth.
Castro followed Baez’s shot in the sixth with his 12th home run. It was the fourth time this season Chicago has hit back-to-back homers.
Hendricks pitched a career-high eight innings after the start of the game was delayed 39 minutes because of rain.