Castillo cranked a two-run blast that cleared the towering scoreboard in left-center field at Isotopes Park, putting him alongside former Isotope Rob Stratton as the only players to put one over the 40-foot structure in a game.
The 24-year-old catcher said he did not know if was the longest home run he had ever hit.
"I hit one last year in Reno that went pretty far, [but] I don't see the home runs," Castillo said. "But my teammates and the other team told me that it was."
Castillo hit two home runs in that game and went 10-for-16 with 12 RBIs over the week to earn Pacific Coast League Player of the Week honors for June 13-19. The big week was the culmination of the Chicago Cubs' top catching prospect's resurgence.
"I've just been working on [my hitting] and seeing the ball well," Castillo said. "I'm just trying to put my best swing on it."
Castillo is making up for lost time after his Spring Training ended early when he suffered a brutal injury for a catcher.
"I got a broken [right index] finger and a torn ligament at the end of camp," Castillo said. "I just gotta rehab it. It's still a little bit sore.
"They told me I've got to play with it, because they can do nothing about it. It's just a torn ligament and a little bit of a fracture of my finger. It's still bad when I get jammed and throw to second, but it's something I have to play with."
Castillo has hit .315 (40-for-127) with 10 home runs and 26 RBIs since rejoining Iowa in May.
"That's an injury that he understands is going to be sore, he's going to have to play through it," Iowa manager Bill Dancy said. "There will be days that I told him if it's really bothering him to come in and let me know and we'll give him the day [off]. But he's done a good job with it. If he gets a fastball, he can take it a long way."
Castillo struggled in his first season at Triple-A in 2010, batting .255 (61-for-239) with 13 homers and 59 RBIs. His age played a part in his performance, Castillo said.
"Yes, I think so, because my first year I was 22, 23," he said. "I just came here and I see everything is faster, people I'd seen play in the big leagues. At the end of the year, it was the same baseball I've played everywhere else. I tried to do my best. This year I feel more comfortable because I know the league, I know the pitchers, I know what to do to help myself."
Rally Sounds: Nashville entered the bottom of the ninth trailing New Orleans on Saturday, 3-0, only to pull out a dramatic victory. The Sounds' first two batters reached on a walk and a double, followed by Brett Carroll's two-run double and Mat Gamel's walk-off, two-run homer to right-center. It marked the fourth time this season that Nashville rallied after trailing in the eighth inning or later.
Mix and match: The Tucson Padres have featured just a few different looks in their first season. Through the first 75 games of the season, the club has used six different catchers, eight second basemen, nine shortstops, 10 left fielders and 12 designated hitters.
Welcome back: Salt Lake right-hander Jerome Williams picked up his first Triple-A victory in two years Saturday, allowing just one earned run in six innings as the Bees defeated Fresno, 7-5. The one-time San Francisco Giants prospect did not pitch in 2010. He signed with the Angels organization this month.