Braves catcher Gerald Laird had pounced on the ball and threw to Walden, who tagged Castro. Cubs manager Rick Renteria challenged the call, but it was confirmed after an 89-second review.
"[Laird] made a [heck] of a play on that ball that got away," Braves manager Fredi Gonzalez said. "That was a big play, believe it or not. That was a big out at the plate."
Catcher John Baker then followed with a two-run double to make it 10-7. Walden retired Junior Lake to end the inning.
Earlier, Julio Teheran and the Braves avoided a possible jam in the first inning when they successfully challenged a call at second.
Chicago left fielder Chris Coghlan led off with a walk and then attempted a steal. He was called safe by second-base umpire Brian O'Nora, but Gonzalez challenged the call and it was overturned for the first out.
The next Cubs batter, Arismendy Alcantara, doubled, which would have likely scored Coghlan from second for the first run of the game. But instead, Teheran was able to work out of trouble and strand Alcantara.
Daniel Kramer is an associate reporter for MLB.com. John Jackson is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.