Guillem de Castro was a notable Spanish playwright and poet from the late 16th and early 17th centuries. Born in Valencia around 1569, he is recognized for his contributions to the Spanish Golden Age of literature. His works often reflect the themes of love, honor, and the complexities of human relationships. De Castro is best known for his play "El conde de Salvatierra," which showcases his talent for blending drama with poetic elements. He also played a significant role in the development of Spanish theater, influencing later playwrights such as Lope de Vega and Tirso de Molina.