The history of Tabaristän, from the late Sassanid era through the middle Islamic age, underwent significant social and political changes the result of which were independence seeking movements leading to some semi-independent local governments. Each of these governments claimed political and social governance over a particular region. Castles, as important and influential military bases, were means at the disposal of the commanders and governors to maintain their sociopolitical power. Among the known castles in the mountains of Mazandaran province, Pouläd Castle in Nour, the capital of Nour County, is one of the symbols of territorial defense. It is located on a tall mount off the city of Baladeh in Nour County. Being built on an elevated location and overlooking the important surrounding regions and strategic ways has made Pouläd Castle one of the important centers in the sociopolitical changes. The present study follows a field work method as well as a library research method to investigate the defense structure of Pouläd castle and its geopolitical and geostrategic role in the power distribution pattern over the region of Rouyän in Tabaristän during the Islamic middle ages. The results show that the builders had tried their best to enhance the stability of the castle by building several layers of walls, utilizing the slope of the mountain, building several defense towers, and using rocky and impervious points as natural forts. Architectural analysis shows that Pouläd Castle, with both its military and political functions, has had a prominent role in the political changes of western Tabaristän.