In the Ryukyu Islands, gusuku (グスク, 御城; Okinawan: gushiku) are castles and fortresses that feature stone walls. The origin of gusuku remain controversial. The Gusuku period of the islands is the archaeological period in which most gusuku are thought to have been built; it follows the shell-mound period and precedes the Sanzan period. Many gusuku and related cultural remains on Okinawa Island have been listed by UNESCO as World Heritage Sites under the title Gusuku Sites and Related Properties of the Kingdom of Ryukyu.

The Yarazamori Gusuku Inscription (1554) contains phrases, "pile gusuku" (くすくつませ) and "pile up gusuku and ..." (くすくつみつけて); apparently, gusuku in these phrases refers to stone walls. In the Omoro Sōshi (16th–17th centuries), the term gusuku is written as "くすく," or "ぐすく" in hiragana. Occasionally, the Chinese character "城" (castle) is assigned to it. In later ryūka and kumi odori, the reading shiro is also used for the same Chinese character, in addition to also using 城内 (shiro-uchi; inside the castle). The references to gusuku in the Omoro Sōshi are mostly about castles and fortresses, but sacred places and places of worship are called gusuku as well. In some cases, gusuku simply refers to Shuri Castle. The Liuqiu-guan yiyu (琉球館訳語), a Okinawan dictionary written in Chinese, maps Chinese "皇城" (imperial palace) to the transcription "姑速姑" (gu-su-gu). Similarly, the Yiyu yinshi (音韻字海) assigns "窟宿孤" (ku-su-gu) to "皇城."

There is no consensus about the etymology of gusuku. Chamberlain analyzed the word as the combination of gu (< honorific go 御) and shuku (宿). Kanazawa Shōzaburō also segmented into gu and suku but considered that the latter half was cognate with Old Japanese shiki, in which ki was a loan from Old Korean. Iha Fuyū proposed that suku was cognate with soko (塞, fortress). Hirata Tsugumasa considered that suku was cognate with Japanese soko (底, bottom). Similarly, Higashionna Kanjun raised doubts over the analysis of gu since older records always used honorific u (< o) instead of gu (< go). Nakahara Zenchū identified gu as go (stone).

The most prominent feature of most gusuku is their wall(s). Gusuku walls are primarily made with Ryukyuan limestone and, sometimes, coral. There are three types of gusuku walls: aikata-zumi, nozura-zumi, and nuno-zumi. Examples of each are Nakagusuku Castle, Nakijin Castle, Zakimi Castle, and parts of Shuri Castle. The shape of gusuku walls usually follows the contours of the land. They are usually thick, and sometimes have low parapets atop them. Some gusuku walls, like those of Nakagusuku Castle, were designed to resist cannon fire.

Gusuku have one or more baileys. The baileys of typical gusuku usually contained a residence, a well, an utaki, and storage buildings. Larger gusuku, like Nakijin Castle, could have more than five baileys, while smaller gusuku, like Iha Castle, had a single bailey.

Gusuku have one or more entrances, often guarded by a heavy gate or gatehouse. Gates were the strongpoints of gusuku. Many gusuku, like Nakijin Castle, were adapted to have gun ports next to their gates.

At the heart of most gusuku was the Main Hall (正殿, Seiden). The Main Hall was typically the residence of a feudal lord (Aji). The palace at Shuri Castle is the most prominent Main Hall, being the only one remaining, but the site of the Main Hall is very obvious at other gusuku, such as Katsuren Castle.

Almost all gusuku contain or are near an utaki (御嶽), shrines and sites of religious importance in the Ryukyuan religion. The relationship between utaki and gusuku has led some experts to question the origin and essence of gusuku.