Within the archives of Beaumont Castle (Archives de l’État à Mons, Belgium) can be found a single encrypted letter to Charles III de Croÿ. As the letter was not preserved alongside its decryption, a cryptanalysis was performed in early 2025. Despite the shortness of the encrypted segments, the cipher key was reconstructed, revealing a rudimentary system for the time: a simple mono-alphabetic substitution, without nomenclature or null characters. Sent from Arras (Spanish Netherlands) on 25 November 1592, the letter lacks unfortunately the sender’s name. Nevertheless it sheds light on the particular context of the Spanish Netherlands at the end of 1592. Its governor, Alexander Farnese, supported the Catholic League, which was at war with the French King Henri IV (French Wars of Religion), but he faced himself a revolt of some provinces (Eighty Years’ War).