In 2332, as a Starfleet Academy cadet, he was assigned to the USS Enterprise-C. After the death of helmsman Aine McAvennie during an attack by unknown aliens, Castillo took the helm position. (ST short story: "Hour of Fire")
Castillo remained on the Enterprise for twelve years, eventually reaching the rank of lieutenant. In 2344, the Enterprise was caught in a battle with the Romulans, and was sucked into a temporal vortex and deposited 22 years in the future. Intercepted by the USS Enterprise-D, he met Lieutenant Natasha Yar, and began a relationship with her. Following the death of Captain Rachel Garrett, Castillo was the last surviving senior officer. He and Yar took the Enterprise-C back into the past to defend the Klingon world of Narendra III. (TNG episode: "Yesterday's Enterprise")
- The novel Vulcan's Heart has Castillo taken captive by Romulans, and killed shortly thereafter by a Romulan commander for insolence. The short story "The Fourth Toast", however, has Castillo still alive into the 2360s.
The temporal vortex instead deposited them in an alternate 2409 wherein the Klingon Empire had conquered the Federation in the 2360s and was in turn conquered by the Dominion. The Enterprise was captured by the Tholian Assembly and the surviving crew enslaved. They eventually escaped with assistance from other prisoners and the Wells-class timeship USS Pastak and returned with their ship to 2344, where the ship was destroyed defending Narendra III against the Romulans. (STO mission: "Temporal Ambassador"; TNG episode: "Yesterday's Enterprise")
|USS Enterprise-C personnel|
|Alfonzo • Aria • Aristide • Bat-Levi • A. Batra • Bulast • Carmona • Casado • Castillo • Chu-Fong • Cohen • Coron • Donald • Fredericks • Garrett • Thule G'Dok Glemoor • al-Halak • Hemachandra • Holmes • Junior • Katie • Kepler • Kodell • LoPresti • Luttrull • McAvennie • Nee • Norrie • Parker • Singh • Singh • Stachow • Stern • Stewart • Thithta • Tholav • Tyvan • Varani • Vinson|