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 What are you talking about? What you did is the fill-in-the-blank question:
Gusuku ... often refers to castles or fortresses in ______ that feature stone walls.
 And your answer is ""the Ryukyu Islands."" That's wrong in the sense that we wouldn't say, ""We speak Germanic"" when we actually speak English. Castles or fortresses that feature stone walls are typical of Okinawa. Applying the term to other island groups is highly problematic to say the least.
 Narratives about gusuku are truly chaotic. No wonder you don't understand the situation correctly. No one can define the meaning of gusuku, which reminds me of family resemblance. What's why I avoided the ""A is B"" definition and used ""A often refers to B"" in the article. Explaining many conflicting theories as I did in the article is confusing. So I present just one hypothesis which I personally support:
 The word gusuku originally meant stone walls that, say, partitioned agricultural fields. In other words, the word gusuku was not associated with castles. This meaning survives in Yaeyama.
 There has been a long tradition of mountain cult. At first, the word gusuku was not associated with mountain cult.
 In Amami, medieval mountain fortifications, that are typical of mainland Japan, were constructed, probably in response to Ryūkyū's repeated invasions. At this stage, the word gusuku was not associated with these mountain fortifications.
 In Okinawa, some sacred mountains were later fortified with stone walls. One consequence of the fusion of mountain cult and fortification was the association of the word gusuku with mountain cult and/or fortification. The association of gusuku with stonework was weakened, but anyway Okinawa's castles and fortresses had stone walls.
 Ryūkyū finally conquered Amami and introduced the noro ritual system to Amami. The new meanings of gusuku (1. related to mountain cult, 2. fortification) were also introduced to Amami. Most of the former medieval mountain fortifications were abandoned, but some were transformed into sacred places and given the name of gusuku even though they lacked stone walls. Some people of Amami might have interpreted the word gusuku as fortifications regardless of whether they had stone walls. Others reinterpreted the word as a sacred place regardless of whether it had stone walls. For this reason, some gusuku are on sand dunes, on cliff edges, and in caves.
 Archaeologists of Okinawa Prefecture re-used the word gusuku as a technical term (yet another meaning). This was the source of confusion. They referred to Okinawa's castles and fortresses that feature stone walls as gusuku. They met fierce oppositions from folklorists because their use of gusuku was not grounded in people's actual usages.
 Some archaeologists of Okinawa Prefecture attempted to extend the gusuku-as-fortifications framework to Amami, Miyako and Yaeyama, regardless of whether local people referred to the target as gusuku (The remains of human settlements of Yaeyama are not called gusuku by local people either). They met fierce oppositions not only from folklorists but also from fellow archaeologists.
 It is convenient to consider a dichotomy between inductive and deductive reasoning. Linguists, folklorists and ethnologists base their arguments on people's actual usages and try to infer a generalized conclusion from them. By contrast, archaeologists often define a term, independently of people's actual usages, and apply it to those which meet the definition. Due to lack of written materials, archaeologists need to name archaeological findings by themselves. So they naturally clash with scholars from other disciplines.
 It is important to note that the gusuku-as-fortifications framework (that's what the lead sentence says) is no longer supported in the archaeology of the Amami Islands. The renaming of so-called ""Uragami Gusuku"" was a symbolic event. A gusuku is a gusuku only because people refer to it as such. It's not because the referent meets some definition. In Amami, gusuku (as a technical term) is dead, long live gusuku (as to people's actual usages)!   "