Tokara Islands
In his survey of earlier studies, Takanashi Osamu criticized Naka's theory because his dating lacked evidence. In fact, gusuku with established dates were mostly from the 14th to 16th centuries. While other archaeologists had focused on mountain fortifications, he paid attention to gusuku in flat land. He also indicated the possible presence of gusuku in the Tokara Islands, which are located to the north of Amami.