Castro and outfielder Brad Snyder were assigned to the Minor League camp after Saturday's 4-0 split-squad win over the Kansas City Royals. Castro hit .423 this spring, Snyder hit .313.
"I felt good with the experience, first time being up here," Castro said. "I'm going to go down there and work hard and try to get back up here as soon as possible."
Castro, who turns 20 on Wednesday, used catcher Geovany Soto as his interpreter, but when asked if he felt he was ready to play in the big leagues, he didn't need help with his answer.
"I'm ready," Castro said. "I'm ready to play in the big leagues."
This was the first time Cubs manager Lou Piniella got to see the young infielder, and he was impressed.
"Castro had a phenomenal spring for a young player," Piniella said. "Nineteen years old, to come in here and look like a veteran and swing the bat ... he showed his athleticism. He came in here highly touted and left here even more touted."
Ryan Theriot doesn't have to look over his shoulder any more. He was batting .533 this spring.
"Maybe having Castro around has something to do with it," Piniella said. "You've got a nice young player like that pushing you."
Give hitting coach Rudy Jaramillo credit. He's tried to get Theriot to stop pulling the ball.
Snyder also left an impact.
"Snyder is a kid who is athletic and can play all three outfield positions," Piniella said. "If we need an outfielder, he'll be competing with a couple kids and whoever is swinging the best will come up here."
The Cubs now have 40 players on the spring roster, although Angel Guzman and Ted Lilly are expected to open the season on the disabled list.
With two weeks remaining, the Cubs have a few spots to finalize. Piniella said there are four relievers competing for one spot in the bullpen, and that group includes John Gaub, James Russell, Marcus Mateo and Mike Parisi.
When asked about Berg, Piniella said: "Berg is in. Berg has done a nice job."
The relievers who are set include closer Carlos Marmol, John Grabow, Esmailin Caridad, Berg, and two of the four pitchers who don't make the rotation. Piniella will pick from Carlos Silva, Jeff Samardzija, Tom Gorzelanny and Sean Marshall.
"I like the arms we have and I like the mix we have," Piniella said.
Piniella said the Cubs have an idea as to which way they're leaning but won't announce anything yet. The staff will sit down after Monday's game to discuss the roster.
With Snyder gone, that leaves Tyler Colvin, Sam Fuld and Jim Adduci battling for the extra outfielder job. The Cubs would like someone who can play all three spots, especially because Xavier Nady is limited throwing while he continues to recover from last year's Tommy John surgery on his right elbow. The Cubs do like both Chad Tracy and Kevin Millar, but it's unlikely they can keep both.
"There's a lot to say about a veteran's contribution and leadership from your bench players," Piniella said. "There's some nice, subtle, important decisions to make here over the next four, five days."
As for the rest of the bench, that's still to be decided. Infielder Andres Blanco has been out with a knee injury, suffered March 6, which hasn't helped his chances.
"I don't know exactly what's going to happen there," Piniella said.
The bench players who are set include Nady, catcher Koyie Hill and infielder Jeff Baker.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.