Catcher David Castillo was groomed to fit into the Athletics system at an early age.

“When I was a kid, I’d get frustrated when I couldn’t get a pitch to hit,” said Castillo. “My dad taught me early on to just take the walk when it’s given to you and take your base. Every since then, I’ve been very aware of my walks and my (strikeout-to-walk) ratio.”

Castillo, 22, has been among the Midwest League leaders all season in two offensive categories the A’s highly covet, runs and on-base percentage, and was hitting .322-5-22 for low Class A Kane County in 33 games.

“He already calls a great game and has established a real professional relationship with many of our pitchers,” Cougars manager Dave Joppie said. “Offensively, he’s just been fantastic. He makes my job very easy–just put him in the three-hole every day and watch him go.”

A 2003 seventh-round pick out of Oral Roberts and the only two-time Mid-Continent Conference Player of the Year, Castillo struggled in his pro debut, hitting .257-1-14 for short-season Vancouver.

“When we signed David, he had so many adjustments to make,” A’s farm director Keith Lieppman said. “He needed to learn our system, adjust to hitting with wood, and deal with a very unfriendly hitter’s park in Vancouver. But we knew he’d improve.”

Listed at 5-foot-9, 185 pounds, Castillo credits his offensive explosion to a stringent workout in the offseason that saw him add nearly 30 pounds. “His work ethic is just absolutely off the charts,” added Lieppman. “When we drafted him, he was a boy. When he showed up in Arizona this spring, he was a man.”

With the rare opportunity to enjoy a major metropolitan area while playing in the minors, Castillo has yet to enjoy the Chicago night life, available just 45 minutes east of Elfstrom Stadium. “I’d love to get out there. My roommate (outfielder Dustin Majewski) and I pooled together and bought a car to get us back and forth from the park, but it’s an old Volvo, like an ’86, and I’m not confident it would make it there.”

Castillo represents one of a number of quality catching prospects to emerge throughout the system in the last two years, as the A’s have turned what was once a system weakness into a sudden strength.

“Over the last several years, we’ve had some prospects that didn’t work out, and we also traded two (Gerald Laird and Miguel Olivo) who are now starting this year in the big leagues,” Lieppman said. “We went from abundance to zero pretty quickly and decided to focus on catchers in the last few drafts.”

That focus is starting to pay off. The most well-known catcher in the system, Moneyball-favorite Jeremy Brown, has struggled this year at Double-A Midland. But John Baker, selected five rounds after Brown out of California, is hitting .315-5-21 while sharing catching duties with Brown and also seeing time at first base.

Lieppman also thinks the A’s have a sleeper in John Suomi, an obscure 22nd-round pick in 2000 out of the College of Cariboo (B.C.). He entered the season with a .253 average and 11 home runs in 213 career games, but is off to a .328-6-30 start for high Class A Modesto in the California League.

The A’s even have backups that they’re high on, including another Canadian, Dave Harriman, who relieves Castillo at Kane County. Harriman received no pro attention as a junior, but was selected in the 17th round of the 2002 draft after beginning his senior season at Armstrong Atlantic (Ga.) State with a 41-game hitting streak, tied for the third longest in Division II history. Harriman is hitting .302-2-8 in 17 games for the Cougars.

“It’s been tough for Dave,” Lieppman said. “He seems to always be in the wrong place at the wrong time, but we like his skills, and we like the fact that pitchers really like to throw to him.”

For Harriman, patience has been the key. “I understand my role and recognize that I’m not going to play every day when we have Castillo here,” Harriman said. “I just wait for my turn to play, and when I get in the lineup, I want to do what he’s doing.”