Slangenburg changed hands many times, usually through inheritance, but twice through sale. The last sale took place in 1895 when the German timber merchant and major industrialist Arnold Passmann from Duisburg-Ruhrort took over the castle and adjacent estate - initially solely because of the large amount of wood that could be cut there. At an auction, Herr Kommerzienrat Arnold Passmann, a German manufacturer and timber merchant, becomes the new owner. He turns the castle into a 'Familienheim' and the castle is used for holidays, family and hunting parties.
Nevertheless, the house was only occupied for three months a year, due to the so-called window tax.
When his favorite daughter Gertrud died in 1901, only twenty years old, he had a family cemetery built. Gertrud Passmann is buried there next to her father and seven other family members.
After the death of his father, the eldest son, Hermann Passmann, continued to manage the Slangenburg. He was very popular with the tenants and during the war years he tried to do everything he could to get the estate, and the people working there, through the war unscathed. He went so far as to send money via Sweden.
It must have hurt Passmann a lot when the Slangenburg was taken from him after the war as "enemy property" and confiscated.