Castro's primary research areas are Spanish and Galician phonology and morphology, and sociolinguistic processes related to minorized languages.
Her first bookAproximación a la fonología y morfología gallegas(1998) was the first in the field to incorporate a generative and autosegmental phonology approach to the study of Galician phonology and morphology. Some of her recent essays published in peer-reviewed journals look at the importance of experiential activities in language learning: “‘Finding the unfamiliar in familiar places.’ The Regis Community-Based Spanish/English Exchange Project: Journeys in place” (2014); and the effects of language policy in language maintenance and change: “The Orality-Literacy Continuum in Galician: Language Choice, Cultural Identity, and Language Policy at a Crossroads” (2015); “The Perceived Presence/Absence of Galician accent on Galician TV Newscasts.” (2017); “Elcontinuumoralidad-“literacidad” en entornos biculturales y bilingües: el gallego y el español nuevo mexicano tradicional”, (2018).
Castro was a founding member of the MLA LLC Forum in Galician Studies (2014) and the head of the organizing committee of the Third North American Symposium of Galician Studies: “Galician Studies Moving West: Galician Language and Culture at the Crossroads” in Denver, CO October 18-20, 2018 co-sponsored by Regis University, Metropolitan State University, and Colorado College.