Cultural geographer Nakamatsu Yashū claimed that the essence of gusuku was a sacred place. His theory was backed by decades of field work that was not limited to the Okinawa Islands but that extended to Amami, Miyako and Yaeyama. He revealed that an overwhelming majority of what were called "gusuku" by local communities did not look like castles or fortresses at all. In fact, they were too isolated from local communities, too small to live in and lacked water supply. Among hundreds of gusuku, only a dozen were fortifications. Each community usually had a gusuku. Gusuku were typically located on hills, but some were on sand dunes, on cliff edges, and in caves. In some communities, what were called gusuku were actually stone tombs. Nakamatsu explained the great diversity of gusuku by one feature in common: sacredness. According to Nakamatsu, a gusuku was in origin a place of "aerial burial." The reason that a dozen of gusuku were transformed into fortress/castle-like structures is unclear, but he conjectured that some rulers had expanded gusuku substantially by building their family residences around them. Shuri Castle, for example, encompasses sacred places such as "Sui mui gusuku" and "Madan mui gusuku", which suggests the original nature of the castle. Archaeologists from Okinawa Prefecture have labeled some archaeological findings as gusuku. Takemoto Masahide claimed that gusuku were defensive communities. He classified what he considered gusuku into three types: According to Takemoto, Type B, which is overwhelming in number, appeared during the transitional period between primitive society and class society. As noted by Asato Susumu, there is a significant gap in the use of the term "gusuku". While Nakamatsu referred to limited space as gusuku, Takamoto applied the term to the whole archaeological site. Archaeologist Tōma Shiichi hypothesized that a gusuku was the residence of an aji (local ruler or warlord) and his family. Since most gusuku in the Okinawa Islands are accompanied with stone walls, he considered that the Gusuku Period was characterized by the formation of class society. Among archaeologists, however, Kokubu Naoichi supported Nakamatsu's theory considering poor living conditions of gusuku. Asato Susumu expressed concern about the association of gusuku with class society because the emergence of political rulers was not well attested by archaeological findings but mostly based on literature that was written centuries later.