Objective Aksu Prefecture in Xinjiang has a large number of ancient city and beacon tower sites in Tang Dynasty, and these military settlements are the product of specific humanistic environment and natural environment. This research aims to sort out the regularity characteristics of the spatial distribution of the aforesaid city sites and beacon towers, so as to provide an important basis for exploring the military deployment in the Western Regions in Tang Dynasty, and provide a physical sample for revealing the military geography of the Western Regions and the relationship between the Great Wall and the Silk Road culture.

Methods This research adopts a combination of qualitative and quantitative analysis methods, considers the architectural heritage of military defense settlements as an element of cultural landscape, and places such architectural heritage in the unique landscape environment system of Aksu Prefecture. The research takes 169 military settlements and beacon tower sites in Aksu Prefecture as the research object, and forms an overall spatial perception of the natural landscape environment and sites in the region through combining the review of historical documents, field investigation of existing sites, and low-altitude data collection by unmanned aerial vehicle. First, the historical and humanistic background of the sites in the research area is analyzed to explore the influence of humanistic factors on the distribution of settlements. Second, the natural landscape environment of the research area is analyzed. With the help of ArcGIS geographic information platform, the elevation and slope values of the three types of settlements are extracted respectively. Meanwhile, the site selection characteristics under the influence of natural factors are analyzed in combination with other natural environment characteristics.

Results Since Anxi Protectorate was set up in Qiuci in the twenty-second year of Zhenguan Period (648) in Tang Dynasty, a large number of fortresses, forts, and other grades of military towns were built in the southern foothills of the Tianshan Mountains and in the northern edge of the Tarim Basin. Among them, a few of the city sites were “Hsing Guo” (nomadic life) and “Cheng Guo” (settled agriculture), which belong to the tie-up prefectural system in Tang Dynasty; most of the city sites were military towns built during the development of the Anxi area in Tang Dynasty, which were obviously hierarchical under the military system. The military system of the “Four Garrisons of Anxi” is: Military town − guard (shouzhuo, a minor military garrison) − town − garrison, with corresponding military cities being military city − guard city − town city − garrison fortress in order. Among the 52 beacon towers in Aksu Prefecture, The Third Cultural Heritage Census confirmed that only 5 beacon towers were built in Han Dynasty, located around the Western Capital of Han Dynasty, which were continuously used in Tang Dynasty; the remaining 46 beacon towers were built in Tang Dynasty. Under the influence of specific natural geography, these city sites and beacon towers gradually form three types of distribution patterns: The northern Tianshan Pass settlement along the southern foothills of the Tianshan Mountains presents a belt-shaped distribution, guarding the north and south valley exits of the Tianshan Mountains as a line of, defense for the northern nomadic tribes; the central Silk Road along the line of the military settlements presents a linear distribution, forming a east-west line of defense through the traffic cordon; the southern oasis garrison settlements are distributed in the oasis plain of Qiuci, forming a large-scale garrison area controlling the northern edge of the Tarim Basin. The three major defense areas centering around Anxi Protectorate form a systematic layout. Under the influence of natural factors, the site selection characteristics of these three types of settlements are as follows: 1) Tianshan Pass Road settlements — defenses are set up in alignment with mountain trend to guard the ancient roads; 2) Silk Road settlements — defenses are set up under the road to protect the Silk Road; 3) Oasis Garrison settlements — local resources are abundant enough to protect the garrison.

Conclusion The sites of military settlements in Aksu Prefecture are important witnesses to the history of Xinjiang. At present, most of the cultural heritage protection work in the region is concentrated in the fields of Buddhism and Qiuci culture, and there is still much room for the excavation of the value and protection of military heritage. Moreover, the research finds that a large number of sites are poorly preserved and continuously damaged under the influence of natural weathering and other factors. This research explores the influence of such factors as natural environment, military strategy, Silk Road trade, settlement and border guarding on the formation and distribution of the sites of military settlements, and summarizes the systematic layout mode and site selection type of military settlements in Tang Dynasty in the research area. The research believes that the systematic layout of the military defense strategy adopted in the ancient Western Regions is of great historical value. With the development of archaeological work, the research on systematic layout characteristics can provide clues for the speculation of the location of potential city sites, beacon towers and other military sites, so that the gradual recovery of military defense patterns in the Western Regions in Han and Tang dynasties becomes possible. Meanwhile, this research may also provide a basis for the digital protection of military heritage, and provide a theoretical reference for the holistic protection of military culture and Silk Road culture in the ancient Western Regions and the protection of natural and humanistic landscape in the desert oasis area.