This study examines the spatial structure of Jeju-eupseong (Town-castle) in the Joseon Dynasty. The spatial structure of Jeju-eupseong has distinctive features and regional characteristics that are substantially different from typical eupseong in inland regions. This is mainly attributed to the geographical characteristics of the island and the administrative system with strengthened military functions. Jeju-eupseong displays a distinctive layout of major buildings and road system with a plaza located in its center in comparison with the ones found in inland regions. Gwandeokjeong Plaza in Jeju-eupseong served as the center of Jeju’s politics, administration, and culture during the Joseon Dynasty, and it, in the modernization period, has become an open public space in the heart of Old Downtown used for community gatherings. Therefore, this study aims to explore the historical background and distinctive spatial structure of Jeju-eupseong in comparison with eupseong in other parts of Korea. Moreover, the study examines the characteristics of Gwandeokjeong as a plaza that has not been dealt with in other studies of eupseong.