High Desert Mavericks
During the 2015 season, Castillo pitched in the Texas and Toronto organizations, seeing time with Class-A High Desert Mavericks, Class-A Dunedin Blue Jays and Double-A Frisco RoughRiders. In 27 games, including one start, between the three teams, Castillo posted a 2–2 record with a 4.78 ERA and 39 strikeouts.