Joey Votto went deep for a Reds-record sixth game in a row. That includes his six homers in the four-game series against the Cubs that Cincinnati wrapped up with a 7-4 win Thursday.
Votto's 20th homer of the season was a two-run shot in the first, when he hit a fastball from Alec Mills (4-4) into the center field bleachers.
Chicago went ahead 3-2 lead before the Reds roughed up Mills in the sixth to go ahead to stay. After loading the bases with no outs, Tucker Barnhart drove in two runs on a soft groundball that went through the middle of the infield for a 4-3 lead.
Cincinnati added three runs off the Cubs bullpen in the seventh, with Kyle Farmer and Aristides Aquino each driving in a run and a Chicago error allowing a third to score.
The Cubs scored three runs off Castillo (5-10), who had only given up three runs total in his previous four starts. Patrick Wisdom hit his 16th home run in the fourth, and Wilson Contreras belted a two-run shot in the fifth.
Castillo finished with eight strikeouts in six innings, allowing seven hits.
Ian Happ hit a home run in the ninth off Reds reliever Heath Hembree.