The purposes of this study is to investigate spatial structure characteristics of earthen walled fortresses based on topography, which are locational and Plane morphology, Cross sections of earthworks. Target areas of this study are 37 earthen walled fortresses which are designated as cultural properties. This study is conducted with methodologies of literature review, field investigation, investigation on topography materials and relation analyses of topography&spatial structure. The results from study on spatial structure of earthen walled fortresses are as follows. First, Earthen walled fortresses on flat topography were adjacent to water systems like rivers or streams. Plane morphology showed regular shapes. The cross section morphology presented trapezoid the most with earthworks of which outer and inner walls were built. Second, More than 70% of earthen walled fortresses on hilly topography had water system within close range of approximately 1.5km. Plane morphology was mainly with more than 73% of irregular types. Cross sections of earthworks were of morphology with more than 86% of bordered type and another type leaning against existing topography like mountains. Third, 59% of earthen walled fortresses in mountain areas had water system within their close range, which indicated that they depended on external water system less than those with hilly topography. Plane morphology was mainly with more than 67% of irregular types. Cross sections of earthworks were of monoslope method with more than 94% of bordered type and another type leaning against mountains.