Castro was among nine players told by manager Brad Mills and general manager Ed Wade that they weren't going to make the Opening Day roster. Castro and right-hander Casey Daigle were reassigned to Minor League camp, and right-hander Wilton Lopez and utility man Edwin Maysonet were optioned to Triple-A Round Rock.
In addition, non-roster invitees Gary Majewski (right-hander), Gustavo Chacin (left-hander), Drew Meyer (infielder), Chris Shelton (first baseman) and Kevin Cash (catcher) were reassigned to Minor League camp, but will remain with the team through this weekend's exhibition games in Houston. Daigle will remain with the team until after he pitches in Thursday's Grapefruit League finale.
"They have done a great job of leaving a good impression with not only myself, but the whole coaching staff and the front office as well," Mills said.
The roster moves leave the Astros with 29 players in camp, meaning four more cuts will have to be made before Sunday afternoon's deadline to set the 25-man roster for Opening Day. Considering two of the 14 pitchers remaining in camp are injured, right-hander Sammy Gervacio has likely made the team.
"I can't remember a spring where we've had this many guys in camp this late," Wade said. "Some of it is attributed to the fact we do have some guys banged up, but, more importantly, the level of talent and the way so many guys put their best foot forward is the reason why. We moved nine guys out of camp today, and we're still not over. It's been good."
Towles, who won an Opening Day roster spot in 2008 before his struggles at the plate bought him a ticket to Round Rock, appears to be the starter, though Mills and Wade said no starter has been named. Towles said he was told he would get the lion's share of catching time.
"I think they said I was pretty much going to be the everyday guy," said Towles, who hit .359 with two homers and nine RBIs in 36 spring at-bats.
"What I've seen on film and from talking to people, he seemed to improve," Mills said of Towles. "The way he's gone about his business has been successful. He's got experience at the Major League level, and I think the way he's handled himself this spring has been very good. We're looking forward to the things he has to offer and bring to the ballclub."
Castro, the club's No. 1 Draft pick in 2008, hit .309 with seven homers and 44 RBIs last year at high Class A Lancaster and .293 with three homers and 29 RBIs at Double-A Corpus Christi. He will start the year in Round Rock, marking his first appearance in Triple-A.
"He did everything we asked," Mills said. "He showed why he is the prospect he is. There's no reason to think [with] his performance this spring he's not going to be a solid Major League catcher for a long time."
Wade said the club is happy with its assortment of catchers.
"We're not out looking for catching," he said. "Before that becomes misinterpreted as maybe we're not satisfied with what we have in camp, we're more than satisfied in the context to have to send Jason Castro out. Millsie wants to wait to set his lineup, and I don't want anybody to misunderstand or [think] there's a hidden meaning or that we're trolling for a catcher. That's not the case."
Right-handers Alberto Arias and Yorman Bazardo are hurt, leaving the Astros with 12 healthy pitchers, including Gervacio. The presence of Brian Moehler, Felipe Paulino and Bud Norris suggests that Moehler will start the season in the bullpen.
The club needs to trim one outfielder, and non-roster invitee Jason Bourgeois has been out of action for more than a week because of a strained hamstring. Cory Sullivan, the only other non-roster invitee remaining in camp, is all but assured of getting the final outfield spot.
The only remaining drama remains on the infield, where one player needs to be cut. That will likely be Chris Johnson, unless Lance Berkman begins the season on the disabled list. Johnson had a great spring at the plate, but Pedro Feliz is entrenched as a starter and the Astros want Johnson to play every day. Berkman, who had knee surgery March 13, had a setback Tuesday when he said the knee was swollen and he couldn't run as he had hoped.
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.