When the frontier lost the Great Wall: The spatial planning of the Northern Song Northwest Fortress System

Based on spatial data on fort elements from the historical–geographical database of the Northern Song Northwest Fortress System, this study used an integrated method combining historical document analysis with GIS spatial correlation analysis involving mutual interpretation and corroboration. It reconstructs the spatial distribution structure, functional characteristics, and operational mechanisms for this military defense system at macro-geographical scales. The study reveals that the Northwest Fortress System had a composite spatial structure characterized by a homogeneous military support network superimposed on a radially distributed military–agricultural settlement network. Through historical–geographical analysis of typical localized structural variation, the paper systematically elucidates the complexity and diversity of this defensive complex. A preliminary comparative analysis examined its fundamental spatial structural differences from the Great Wall system in ancient China. This research establishes valuable perspectives for deepening our understanding of the operating mechanism, formation background, and historical roles of mega-scale military engineering systems in Chinese history, including the Great Wall systems and fortress-garrison systems across various dynasties.