This study examines the relationship between geographic variables and the location, architecture, and long-term occupation of the Ismaʿili fortresses of Dāmḡān County in northeastern Iran. During the Islamic period, Dāmḡān formed an important sector of the Ismaʿili defensive network in the historical region of Qūmis, containing several major fortresses and outposts, including Gerdkūh, Mehrnegār, Mansūrkūh, Qaḷʿa-ye Doḵtar, Stīvhād, and Shīr. The primary objective of this research is to evaluate the extent to which environmental and geographic factors influenced the selection of fortress locations and the persistence of occupation at these sites. The study employs a descriptive–analytical methodology that combines archaeological field survey, historical and textual evidence, satellite imagery, and Geographic Information Systems (GIS) analysis. A range of environmental variables—including elevation, topography, slope, hydrology, climate, geology, mineral resources, vegetation, and communication routes—were examined and compared with the spatial distribution of the Ismaʿili fortifications. The results demonstrate that the distribution of the fortresses followed a deliberate and non-random pattern closely associated with strategic environmental conditions. Most sites were established on elevated terrain overlooking the Čašma-ye ʿAlī River corridor, major communication routes connected to the Great Khorasan Road (Silk Road), agriculturally productive plains, and mineral-resource zones. The fortresses also maintained visual interconnectivity and benefited from favorable defensive topography, reliable water supplies, and access to economic resources. Architectural evidence further indicates that these sites were adapted to local environmental conditions through the construction of cisterns, defensive walls, towers, and multi-level architectural complexes. The study concludes that geographic variables played a decisive role in the location, architectural organization, and long-term viability of the Ismaʿili fortresses of Dāmḡān. These fortifications constituted an integrated territorial network designed to support the military, economic, administrative, and religious objectives of the Nezārī Ismaʿili state in northern Iran.