Chris Coghlan doubled just out of the reach of left fielder Kristopher Negron with out in the 11th inning and scored the winning run on Castro’s single to left-center off reliever Burke Badenhop (0-2).
Chicago took three of four in this series.
Castro’s RBI single in the ninth gave the Cubs the win Saturday night.
Billy Hamilton had a career-high five stolen bases _ the most in a game since Jacoby Ellsbury stole five on May 30, 2013, and also tied a Reds record. Hamilton leads the majors with 31 steals.
Brian Schlitter (1-2), called up from Triple-A Iowa before the game, got two outs in the 11th for the victory.