Castle Dean, also called Castle Dounne, was built as a home for the factor, or superintendent of Dundurn. Apparently this man was named Dean, but this fact has been disputed. Some sources say the building was not even called Castle Dean until it was remodeled in the 1908. Other sources say the home was called Castle Dounne, presumably after Doune Castle in central Scotland. (The castle from the movie Monty Python and the Holy Grail).
Early records show the building as being named St. Marys Lodge after it was built in the 1830s. (same time as Dundurn Castle). Originally, it was much smaller. When it was remodeled in 1908 by a man named Robert Anderson, the turret and rounded corner bay section were added in the south, giving it the look of a castle. This is probably when it took the name Castle Dean.
In Brian Henleys book The Grand Old Buildings of Hamilton, he says that shortly before World War 1, the house was sold to a prominent Hamilton dairy company. This implies that it was used in some commercial capacity for a period before being converted back into a residence sometime before 1947.
Much of this information was found in this Hamilton Heritage Committee document: Hamiltons Heritage Volume 5 - Reasons for Designation under Part IV of the Ontario Heritage Act