The publisher in Antwerp who continued to be responsible for the publication and the diffusion of an important part of De Castro's music, was Jan Moretus, who took over the publishing office of his father-in-law Christopher Plantin in 1589.
However, the Plantin-Moretus publishing house is never mentioned on any of De Castro's music prints, while the firm was mentioned on editions of other composers (Philippus De Monte, Jacobus De Kerle, ...). Two letters now preserved in the Plantin-Moretus Museum in Antwerp show that the role played by Moretus (and presumably by Plantin before him) in De Castro's career was much more influential than had been suspected until now. The two letters are written by Jean De Castro in 1589 and 1591 and addressed to Jan Moretus.
They uncover some of the mechanics at work behind the publication of De Castro's music books. Apparently, the composer first sent his music manuscript to Jan Moretus, who read and if necessary, corrected them, and then sent the manuscript to the printer Phalèse. Part of the printed copies returned to the Plantin bookshop where they were sold to other book merchants and music lovers from the Low Countries and abroad (see also the diffusion of De Castro's music through the Plantin bookshop).