The Spring Castle is located at Rock Island State Park across from the Great Falls Cotton Mill. Here is the text of the historic marker: The Spring Castle is an elaborate spring house built while the textile mill was in operation. The reservoir behind the wall contains water from a natural spring. This provided water to the mill and surrounding houses for everyday use as well as fire protection for the machine shop that once stood across the road and connected to the textile mill. Fire protection was extremely important since fires at textile mills were very common with such large hot machinery in close proximity to flammable materials.