Richard posted several shots of this Kalamazoo landmark to the new Historic Michigan pool on Flickr. One of his shots is of the Michigan Historic Marker which tells us it was built in a neighborhood designed by Kalamazoo military regalia manufacturer Frank Henderson.
The Kalamazoo Public Library goes into a lot more depth (and has a cool old photo of the castle):
The Queen Anne style house–always called “The Castle” by local residents because of its ornate style and imposing hilltop location–was designed by C. A. Gombert of Milwaukee, Wisconsin, and cost $72,000 to build. With seven baths, a thirteen-head shower, an elevator, a third-floor ballroom, and a hot tub on the roof (added later) the 25-room castle exemplifies high society and expensive tastes. The castle’s exterior is made of Lake Superior sandstone and brick, and the interior wood includes mahogany, bird’s eye maple, quartered oak, birch, and sycamore.
Henderson Castle is currently a bed & breakfast (visit Henderson Castle B&B site).