Three music volumes have been preserved from the period when De Castro was chapelmaster at the Düsseldorf court (1588-1591). Since this position seems to have offered insufficient prospects - not in the least because of the increasing insanity of the Duke -, the composer soon looked for employment elsewhere. He dedicated his Düsseldorf volumes mainly to notable figures in Cologne.Apparently, Cologne attracted De Castro strongly. He acknowledged the importance of the city in the preface of his Cantiones sacrae of 1591 by expressing his appreciation for the distinguished intellectual climate, the flourishing musical life, and the excellent schools and colleges that had made the city famous throughout Europe. Moreover, after the economical decline of Antwerp (when the Scheldt closed down in 1585), Cologne offered a new home base for many craftsmen and merchants, some of whom were old acquaintances of De Castro. Finally, a friend of De Castro, the humanist, Jean Polit, was employed at the court of Cologne as historiographer of the Archbishop Ernst of Bavaria.
De Castro dedicated his Düsseldorf publications to:
- Ernst of Bavaria (1588)
- Cesare Homodei (1588)
- Herman von Wedich (1591)
The first volume De Castro published as chapelmaster in Düsseldorf, was the Novae cantiones sacrae, quae vulgo motetta vocantur (Douai, Jean Bogard, 1588), dedicated to Ernst of Bavaria.A son of Albert V, Duke of Bavaria, Ernst was Prince Bishop of Liège (since 1581) and was elected Archbishop of Cologne in 1583, a position he maintained until his death in 1612. Under his reign Cologne becomes a powerful Catholic bastion in predominantly Protestant Germany. Ernst was a very "worldly" Archbishop; he loved painting and music and received music lessons from Orlandus Lassus.
Five years later De Castro would remember Ernst with another dedication: the Cantiones aliquot sacrae trium vocum (Cologne, Grevenbruch, 1593).
However, these dedications and his relation with Ernst's historiographer Jean Polit notwithstanding, De Castro's attempts to enter the service of the archiepiscopal palace seem to have failed.
In 1588 Jean De Castro addressed his first volume of madrigals to Cesare Homodei.Homodei was a merchant from Milan who resided in Antwerp (where he may have met De Castro), until he moved to Cologne in 1585. Next to his commercial activities, he displayed a big interest in music. Not only De Castro wrote music for him. In 1583 Andreas Pevernage offered him a collection with madrigals, including the work of twenty-nine polyphonists: the Harmonia celeste, which became a bestseller at the Plantin bookshop.
In 1588 the Milanese merchant was honoured with the dedication of a book covering the history of Liège, written by the Liège poet Jean Polit.
The last music book De Castro composed in Düsseldorf appeared under the title Rose fresche. Madrigali novi ... a tre voci. He had the volume published in Venice at the printing house of Ricciardo Amadino and put it under the protection of Herman von Wedich, a Cologne city councillor who maintained commercial ties with Venice and thus may have acted as a contact between the composer and the printer. He was the brother of Theodor, canon of the Cologne church of Saint Gereon to whom some years later De Castro would address his sole volume of two-part motets (see Music Fragment 4).