Objective The Great Wall is a great military project in ancient China. The topographical characteristics in different sections of the Great Wall are skillfully integrated with the artificial fortifications nearby to form a unique geographical landscape. In this research, the geographical landscape of military settlements along the Ming Great Wall is specified as the Great Wall military settlement landscape with an ideal geographical situation based on the cognitive results of military hierarchy and natural geographical environment. The sitings of the Great Wall and nearby military settlements are determined in combination with military and physical geography. At the macro level, the control of agricultural land resources and the occupation of precipitous mountain areas influences the location of the Great Wall; at the micro level, the control of significant water and land routes influence the location of military settlements.

Methods From a macroscopic perspective, the ancients used the term "Xing Sheng" ("favorable geographical position") to describe the characteristics of the geographical environment and the importance of military status on a large scale. From a microscopic perspective, the terms "Chong" ("critical") and "Huan" ("noncritical") were used by the ancients to describe the marching speed, and to distinguish the strategic importance and access difficulty in military geography and the smoothness in the passage of enemy troops. In order to reveal the ancients' wisdom in construction and site selection, and to analyze the Great Wall's military geography and cultural connotation, this research, based on the historical data about the military settlements along the Great Wall in terms of "Xing Sheng" and "Chong Huan", reviews in detail the "nine towns" described in relevant historical records and annals in Ming Dynasty, and has military hierarchy and military geography skillfully integrated through the microscopic concepts of "Chong" and "Huan". In addition, this research selects a number of representative historical works such as the Illustration of Nine Frontier Towns (pronounced as "Jiu Bian Tu Shuo" in Chinese), the Annal of Four Towns and Three Passes (pronounced as "Si Zhen San Guan Zhi" in Chinese), and Illustration of Xuanfu, Datong and Shanxi Towns (pronounced as "Xuan Da Shan Xi San Zhen Tu Shuo" in Chinese) for historical combing and research. On the basis of the documentary research results, the research conducts a spatial analysis of existing military settlements along the Great Wall with recorded "Chong Huan" levels to figure out whether relevant spatial information can be matched with relevant historical descriptions on paper, for which relevant historical, social and natural geographical factors such as distance from the border wall, level of defense, elevation, slope and undulation are selected.

Results The research adopts the ArcGIS platform to quantify and analyze the characteristics of military settlements described in the Illustration of Nine Frontier Towns, the Annal of Four Towns and Three Passes, and the Illustration of Xuanfu, Datong and Shanxi Towns. It is found that the distance from the Great Wall, slope and undulation are the main factors influencing the classification of military settlements, while the distance from the water system and absolute elevation is less influential. In the Illustration of Nine Frontier Towns, the distance from the Great Wall is the dominant factor. The gentler the terrain is, the closer the distance from the Great Wall and the denser the distribution of military forts will be; the further the distance from the Great Wall is, the lower the "Chong Huan" level and the larger the scale will be. In the Annal of Four Towns and Three Passes, slope and undulation are the dominant factors. The narrower a channel is, the fewer the number of enemy troops passing through the channel and the lower the "Chong Huan" level will be. This description reflects the ancients' interpretation of natural geography and observation and judgment of military geography. In general, the "Chong Huan" level of a military fort is precisely divided into four modes: Jichong ("extremely critical") or Zuichong ("most critical"), Cichong ("less critical")or Chong ("critical"), Youcichong ("much less critical") or Shaohuan ("slightly noncritical"), and Huan ("noncritical"), which historical records vary from different periods. The quantitative graphic interpretation of "Xing Sheng" and "Chong Huan" will help promote the comprehensive research on and reveal the value of the Great Wall's cultural heritage.

Conclusion This research reveals the ancients' view of the planning superiority and important layout of the nine towns from an overall perspective and the siting view of "adapting to local conditions" from a micro perspective, which may provide a reference for the research on the Great Wall's value and the display strategy of national cultural parks.