In their entry about Castle Rock, Hunt’s Guide to the UP says that Castle Rock is a limestone stack, eroded by water and wind to form a “castle” nearly 200 feet above Lake Huron:
Clarence Eby, a St. Ignace photographer and pioneer of area tourism, acquired Castle Rock and, in 1928, opened it as a destination, just as somewhat better area roads enabled motorists to go sightseeing in outlying areas. He made postcards of sights in Mackinac County, the island, and the Straits, created a guidebook with ads from resorts and cabins, and worked to create a Chamber of Commerce information center in town. Today Eby’s grandson Mark runs Castle Rock
Click the link above to see one of Eby’s colorized postcards and get more info including a panoramic photo from Wikipedia.