Castillo is throwing regular bullpen sessions but will "take it slowly" during spring training, AJ Cassavell of MLB.com reports.
Castillo has thrown only two-thirds of an inning since his debut 2018 campaign due to a variety of injuries, so it's logical that the Padres will not rush him back into action. That said, manager Jayce Tingler described Castillo as "full-go," so the left-hander could play a key role in the team's bullpen if he's able to stay healthy.