The campaign is part of the Cubs' efforts to promote a ticket package. There is another billboard with Chicago's Carlos Marmol and St. Louis' Albert Pujols.
Marmol makes sense; he's the Cubs' closer. Castro, who turns 21 on March 24, is entering his sophomore season and he'll be the first to tell you he still has to win a spot on the 25-man roster.
"It was good," Castro said of his first big league season, "but I'm working to be better and be consistent."
The Cubs are doing their part. Infield coach Ivan DeJesus, who was Castro's interpreter Sunday, spent one week in the Dominican Republic working on the shortstop's throwing mechanics to make sure he doesn't get lazy with routine plays. Minor League infield coordinator Franklin Font followed that with another week-long session on footwork. Castro was able to apply what he learned right away when he joined Escogido in the Dominican Winter League and played 16 games, batting .317 (19-for-60).
Castro was promoted to the big leagues May 7 from Double-A Tennessee and he made headlines by hitting a three-run homer in his first at-bat against the Reds. He finished with six RBIs in the game. But he also made 27 errors in 125 games. What does he have to work on?
"Defense, and that's it," Cubs manager Mike Quade said. "Every aspect of defense. I believe he's going to be a good offensive player. I know what we need from him defensively -- we need to be better up the middle and obviously he's a huge part of that.
"As far as he and Blake [DeWitt] and [Darwin] Barney and all those kids playing up the middle, [Jeff Baker], we've got a lot of work to do. 'Cassie' is at the top of that list for sure."
Castro, Alfonso Soriano and Aramis Ramirez reported to Cubs camp on Saturday, and Quade jokingly referred to them as the "Three Amigos." The three players share more than a love of their native Dominican Republic. Soriano took Castro into his home last year and taught him the importance of being prepared for games.
"When I was young, I dreamed and thought about playing with them on the same team," Castro said of Soriano and Ramirez. "Now, I have the opportunity to do it and it makes me feel more relaxed and that I accomplished that dream."
As good as his first season was, Castro had a nightmare game Sept. 5 at Wrigley Field. In the seventh inning of an 18-5 loss to the Mets on Sept. 5, Castro forgot how many outs there were. His grounder ended up as a double play when he didn't run to first and was picked off while walking back.
Quade, who was managing his 12th game after taking over for Lou Piniella, benched the young shortstop at the time.
"There's been so much so fast that's so important for this kid and sometimes you think, 'Maybe it's a little too much right now and maybe [sitting] is a good thing,'" Quade said. "It gets taken as discipline, but it really wasn't. I thought it was a teaching moment for the kid."
Castro admits it was a mental mistake. He forgot how many outs there were at that point in the game. He took Quade's decision in a positive way and learned from it.
Now, Castro heads into his second season. DeJesus had a tough time translating "sopomore jinx" when the shortstop was asked about it.
"I know about it, but I don't pay too much attention to it," Castro said. "I'm just concentrating so I can do the best I can."
Quade had a chance to talk to Castro's father on Sunday. The shortstop's parents are here in Arizona with him for Spring Training. They keep him humble. So will the Cubs.
"I think there's enough veteran presence around here, and I know how I am, so he'll remember who he is and how much work he has to do," Quade said. "I don't see him getting all wrapped up in that kind of celebrity or fame. If he does, he won't accomplish all the things we're so excited about him possibly accomplishing and that will be a message that will be driven home on a regular basis from me, or won't, because he won't need it."
A year ago, Castro was in his first Spring Training camp, and dazzling everyone with his stellar play on the field and at the plate. Does he feel any different this season?
"It's the same," Castro said. "I feel like I have to make the team like I did last year. There's no pressure. I don't feel like I've made the team. I have to work hard every day."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.