Castro y Bellvís, Guillén de, 1569-1631, Spanish dramatist, best known of the Valencian group of playwrights of the Golden Age. Three of his plays dramatize episodes from Don Quixote. His masterpiece, Las mocedades del Cid [the youthful adventures of the Cid], is a historical drama that furnished Corneille with the material for his play Le Cid. Castro enjoyed considerable success during his lifetime.