Castro is getting almost universal recognition as the top shortstop prospect in the minors (ESPN, MLB, BA). His walk rate and power are the only concerns raised. Most will be stunned if he is the opening day shortstop but he appears to be headed towards a debut this year.
Castro along with 2009 #1 pick Brett Jackson, Josh Vitters, and Andrew Cashner are among the non-roster invites to spring training.
Looking at the team right now....
Catcher - Soto and Hill are basically a given. The Cubs do have a bunch of catcher prospects in camp this year including Chris Robinson, Steve Clevenger, Robinson Chirinos, and Welington Castillo who all could be MLB catchers in the near future and a could put up a fight for the backup job if given the chance.
Infield - Lee, Ramirez, and Theriot are set. I assume that Blanco is the backup SS this year unless Castro wins the SS job moving Theriot to 2B and backup SS. I wonder if the Cubs will keep Fontenot on the roster if Baker clearly wins the job. Hoffpauir and possibly Nady could back up Lee. Since Hoffpauir was a bit of a disappointment last year, Chad Tracy might be given a shot to take over the backup role to Lee and also should be able to play some 3B so the Cubs are stuck with the backup 2B as their primary backup.
Outfield - Assuming Nady passes his physical, he will join Soriano, Byrd, and Kosuke in what will probably look like a 4 man rotation. Assuming Kosuke is the backup CF, Hoffpauir could be the 5th OF. Fuld is also in the picture.
Starters - Zambrano, Dempster, Wells, and Lilly when healthy. Marshall should be the best option at #5 but Silva, Samardzija, Parisi, and Gorzelany appear to be in the battle that will also provide a #4 starter until Lilly is ready.
Relievers - Marmol at closer with Grabow and Guzman looking right now as the best options at set up. The Cubs appear to still be looking at relievers for that role though. Berg, Caridad, Gaub, Mateo, Parker, and Stevens along with the losers in the #5 starter battle seem to have a chance of sticking in the pen with good springs.