The homer was the third in the past two games for Castillo and his fourth in his past three starts, giving him 11 overall and eight since coming over in a June 3 trade.
"Everybody has a good swing," Castillo said. "When we get in trouble, we chase bad pitches or we swing at bad pitches and everything speeds up in our minds and our bodies. I just feel really comfortable at the plate, seeing the ball good, my timing is good. I just hope to stay like this the whole rest of the year."
After collecting two hits in his first three at-bats, Castillo had a frustrating at-bat in the seventh. With the go-ahead runner on third, Castillo took a called third strike that appeared to be off the outside part of the plate to end the inning.
Castillo would get a shot at redemption when he led off the 10th against Houston reliever Pat Neshek.
"I was really [ticked] because I struck out with first and third my last at-bat," Castillo said of his mindset. "He made a good pitch, couldn't do anything with that pitch. Just tried to calm down and just be me next at-bat. Can't do anything about my last at-bat, so it was just restart my mind, calm down and look for something over the plate that I can hit."
Castillo was 1-for-5 with a pair of strikeouts heading into the at-bat against Neshek, but he drilled the first pitch he saw down the left-field line and into the Crawford Boxes to give the D-backs a 5-4 lead.
"No," Castillo said when asked if he thought the ball was going to stay fair. "Because I hit a little topspin, so I thought the ball was going to go foul before it crossed the yellow pole, so that's why I stayed at home plate a little bit looking at it. But after I saw the ball was going to go fair, I thought it wasn't high enough, so I started running hard."
Castillo began the year with the Cubs, where he did not receive much playing time behind Miguel Montero and David Ross and after being dealt to the Mariners, he found himself stuck behind Mike Zunino.
After starting D-backs catcher Tuffy Gosewisch went down to a season-ending knee injury, Castillo found himself getting the bulk of the playing time and he has responded.
"I think any hitter … the more playing time that you get, I think you're going to get better because you're going to see better pitches and you're going to be able to make the adjustments," Castillo said.