When the Düsseldorf court where he served as chapel master since 1588, declined, Jean De Castro felt compelled to leave his official position. In 1591 he moved to Cologne where he hoped to acquire a job at the court of the Archbishop Ernst of Bavaria with the help of his friend Jean Polit. In view of this ambition, he dedicated several of his new music books to Ernst and to influential figures at the court (including Charles Billehé, Ernst's counsellor to which De Castro addressed his sole volume of masses, and Ferdinand of Bavaria, Ernst's future successor but at that time still coadjutor of the Archbishop; see Table IV nrs. 22, 28 and 30). However, any conclusive evidence of an official appointment of the composer at the Cologne court is still lacking. Until the end of his life he compulsorily remained an "independent" musician and was obliged to look out for a new patron to finance each of his new publications.
Despite his insecure status as "freelance" composer, De Castro's artistic productivity reached its height during his years in Cologne. Out of the twenty-eight preserved volumes published between 1569 and 1599 (excluding reprints and anthologies), no less than sixteen appeared during the eight last years of his life. Chansons continued to form the greatest part of this later repertory, although the share of religious works increased considerably. This special attention to the sacred genre needs to be viewed in the light of the new ambitions of the composer; a great many volumes composed in Cologne were dedicated to Catholic authorities of Cologne belonging to the circles of the archiepiscopal court (Music Fragment 4).