Arrajan (also spelled Argan and Arjan), the metropolis of the Arrajan Khureh, retained a chief political position in southwest Iran during the Sassanian and Islamic periods on the authority of the earliest Islamic historical sources. The merit of exploring the fortifications of this ancient city lies in the fact that, notwithstanding its eminence, the historical evidence lacks not only references to Arrajan’s fortifications but also any hints on the local fort dwellers’ resistances in recurrent occasions of invasion during the five centuries spanning Islam’s arrival in Iran to the Seljuk’s demise, which amounted to the city’s demolition. Archaeologically examining the construction date and the placement of the fortifications forming Arrajan’s defensive system, the role the elements of natural landscape played in the location of the forts, and whether or not the passive defense principles were abided by were the major objectives the present study pursued. Thus, here we attempted to provide answers to the following questions: 1) How did the natural landscape inform Arrajan’s defensive system? 2) To which periods do its forts and defensive and observing fortifications date back? 3) What was the place of its fortifications in the defensive system of the city? Drawing on the sources of Muslim historians and geographers as well as evidences from field surveys, we addressed these topics through a descriptive-historical approach. The results showed that Arrajan’s defensive system at the dawn of the city’s lifespan, i.e. the Sassanian period, was focused principally on controlling the northern and eastern gorges that gave access to the city, to the extent that three forts sat above Tang-e Takab.