Skeletal Frame Light Towers are lighthouse

Lighthouse

A lighthouse is a tower, building, or other type of structure designed to emit light from a system of lamps and lenses or, in older times, from a fire, and used as an aid to navigation for maritime pilots at sea or on inland waterways....

towers that have only an open frame. They are commonly built as aids to navigation; most of them are not considered to be lighthouses. However, during the late nineteenth century and the first years of the twentieth, larger skeletal towers were installed at light stations throughout the United States. These lights were originally tended by keepers, so they were and are listed as lighthouses.

Skeletal towers became very popular with Congress because they cost less than half the price of a stone or brick tower of the same height. Since they were assembled from prefabricated sections, they could be built quickly, even at remote locations. If necessary, they could also be disassembled and shipped to a new location.