Gerhard Grevenbruch was the most important music publisher in Cologne around the turn of the century. De Castro enjoyed substantial promotion from him, since he published no less than six volumes with De Castro's music (see table IV, nrs. 22, 23, 25, 28, 29 and 30). Only the Felmish printers Phalèse-Bellère published more of his work.
Grevenbruch was active as a bookseller, publisher, and printer since 1583. From 1593 onward he revived music printing in Cologne, which had been interrupted for about half a century. Apart from the De Castro volumes, he also issued two voluminous lute anthologies: the Florilegium by Adrianus Denss of 1594 and the Thesaurus harmonicus of 1603 by the French lute player Jean-Baptiste Besard. In 1610 he published two books of chansons by Nikolaus Zangius, one of the principal exponents of the German Lied around 1600. Contemporary Italian music was represented by his reprint (1601) of the popular madrigal comedy La pazzia senile of Adriano Banchieri, originally published by Amadino in Venice, 1598. Although Grevenbruch's interests extended to works by these younger composers, his name is mainly connected with the publication of the later religious music of De Castro (as far as we know, De Castro was the only composer of whose works Grevenbruch published more than two books). This leads to the assumption that De Castro was one of the most established composers, if not the only one, in Cologne during the last decade of the 16th century.