General manager Ed Wade said Humberto Quintero, entering his sixth season in Houston, will fill the backup role, while Towles and Castro duel for the top spot.
Mills said he'll compare everything from their hitting stances to their arm strength to the way they talk to pitchers as he decides who to choose for the job.
"Fact-finding is a good word," Mills said, "just to see what they can do and put it together and go from there. We'll see. Catching, throwing, running a ballgame, handling pitchers, swinging the bat. Everything is going to enter in and be a part of it, not one particular thing."
Towles brings the edge in experience; Castro is considered one of the franchise's most coveted prospects.
"He's going to push me and I'm going to push him and we'll just make each other better," Towles said Thursday.
Towles, 26, is taking his second crack at the starting job after fizzling in 2008. Like Castro, Towles climbed quickly through the Astros' system and he made his major-league debut just three years after playing at the rookie level.
Towles' startling first September only increased the expectations, and may have been more than Towles could handle at the time. He set a franchise record with eight RBIs against St. Louis two weeks after he was called up from Triple-A Round Rock, and he hit .375 in 14 games.
He started the 2008 season as the everyday catcher, but he was shipped to the minors after hitting .145 through 42 games. He appeared in only 16 games with Houston in 2009 and hit .188 in 48 at-bats.
Towles kept his confidence by convincing himself that he was good enough to play in the majors.
"I'd be lying if I said I didn't get frustrated," he said. "But I think everyone gets frustrated if you don't live up to your own expectations. I'm a perfectionist and I expect to do well.
"I just thought to myself, I'm obviously good enough, otherwise, I wouldn't be here," he said. "I just keep that mindset. I know I have talent, and I just try to keep that positive outlook on things."
Towles played for manager Cecil Cooper in all three of his stints with the Astros. Brad Mills replaced the fired Cooper in the offseason and Towles feels like he's got a clean slate.
"Coming in with new coaches and new management, I've got a lot to prove, but I feel like it's in me," he said. "It's a new start for me, so I'm just going to go out there and prove to them that I can play up here."
Castro, 22, played for seven different teams at various levels in 2009, ideal preparation for the demands of a full major-league schedule. He started with the Astros' Single-A affiliate in Lancaster, moved up to Double-A Corpus Christi, then represented the U.S. in the All-Star Futures Game and the IBAF World Cup.
He finished the exhausting year in the Arizona Fall League, where he also played in an All-Star game. He counted more than 600 at-bats (Carlos Lee had 610 for the Astros in 2009), and needed a month just to physically recover.
"It took a toll on my body, and I really learned how to take care of myself and kind of what to expect in the major leagues," Castro said. "No matter what anyone really tells you, it's a learning experience your first year as far as length of schedule and the toll that takes on your body. I feel well-prepared for this year."
Castro hit the weights in the offseason and made a good first impression on Mills.
"He shows up and he's added that weight back on and he looks strong, he looks good," Mills said. "He's got the package, he's put together, but we'll see how it works out."