This paper provides the guidelines that outline the musical activity in the Cathedral of Santa Maria la Mayor in Cuenca, Spain and its implication on the overall development of the Hispanic sacred and polyphonic music of the Late Middle Ages. After a thorough consultation process an archival package has been obtained. Not only are these data unknown under a musicological point of view, but also are these relevant to an intense and important activity that has mainly been observed from Juan Cabeza de Vaca´s Episcopate.