Constructed as a post office and dedicated on July 3, 1898, the French Renaissance Revival architecture of the building was an unusual - if not unique - choice for a post office. The building's architect, William Martin Aitken, claimed that his design was inspired by the early French settlement of the Saginaw Valley.
The building quickly became a cherished local landmark. As the population of the City of Saginaw grew, the building was enlarged to meet increased demands. By the mid-1930s, the building had become inadequate to serve the community's needs, and the federal government proposed construction of a new facility on the site. This proposal met with community-wide opposition and plans were developed to enlarge the existing building. The 1937 addition and reconstruction were inspired by the design of 1896. The Federal Avenue facade was preserved. The extensive additions resulted in the removal of a tower on the South Jefferson Avenue front, new wings on South Jefferson and South Warren and a large mail sorting room on the south side of the building. Almost of the interior spaces were reconfigured and the 1898 lobby was replaced by a new public lobby featuring travertine walls and an oak paneled ceiling.
Thirty years later, a new post office building was constructed on South Washington Avenue, and the South Jefferson Avenue building was renamed the Castle Sub Station. In 1970 a new federal office building was dedicated and the building was threatened with demolition. Again, the community advocated for the preservation of the building, and it was transferred to the County of Saginaw to become the home of the Castle Museum of Saginaw County History.
- Long-term exhibitions on all aspects of local history
- Changing and traveling exhibitions on a variety of topics
- Preserving more than 100,000 objects and artifacts related to historical, cultural and archaeological heritage of the area: furniture, costumes and textiles, vehicles, household goods, toys, photographs, maps and books
- Historical preservation of the Castle Museum and Annex facilities
- Educational programs for audiences of all ages
- Community Tours
- Curriculum-related school programs
- Film presentations
- Family History Room
- Oral History Recording Program
- Research services
- Living History programs
- Active volunteer program