Problem Statement: Qaleh Iraj is one of the Sasanian archaeological sites in Tehran plain. The site is enclosed by arectangular defensive wall (rampart), 1470×1300m in dimension; thus, the surface area of the site would be aound 190ha. So far, field investigations have yielded no extensive architectural remains in this vast area enclosed by the wall. For this reason, the main objective of the present paperwould be to locate the architectural structures related to the living spaces of the inhabitants. In addition, the second query to answer would be the spatial organization of the architectural structures recovered within the Qaleh Iraj rampart. To answer the principal question, it seems, due to the lack of architectural remains in the vast are enclosed by the wall and regarding the fact that such remains are abundant within the defensive wall itself, the inhabitants were probably living within the structures (numerous rooms) built inside the huge defensive wall itself and not in the whole site. To answer the second question about the spatial organization, it seems, according to the field studies, the architectural spaces were systematically builtwithin the entire defensive wall of Qaleh Iraj, including stunning 828 similar rooms, one allembracing corridor, 148 watch-towers, and some exterior arches. Aims: This research was conducted in order to search for the living spaces of the inhabitants of Qaleh Iraj. Due to the fact that they were absent in the whole 190ha area enclosed by the wall, the huge defensive wall was chosen for archaeological excavations. After the discovery of abundant architectural remains within the defensive wall itself, these remains were examined thoroughly. Finally, some hypotheses were offered for explaining the spatial organization of the architectural structures. Methodology: In this research, descriptive-analytical method was used to answer the questions of the study. In the first step, the architectural remains that discovered from the field investigationsweredescribed and then, by putting the jigsaw puzzle pieces together as a whole, archaeological examinationsweredonein order to develop hypotheses on thespatial organization of the defensive wall elements. Results: The presence ofarchitectural remains within the defensive wall or rampart of Qaleh Iraj was unknown previously. Field excavations indicated that the defensive wall itself contained numerous rooms and a corridor and these were built for the sake of living of the inhabitants. As a result of the mentioned archaeological discoveries in Qaleh Iraj, it is now clear that the architectural structures such as rooms were built all over the defensive wall, executed in a similar fashion. The remains within the defensive wall include rooms, a corridor, watch-towers, and exterior arches. In addition to what mentioned, it seems the architectural system was changed in the later phases possibly due to security reasons.