Abstract
Greater Khorasan, which is now divided into six countries, in addition to its geopolitical importance, have great historical importance due to the formation of the Parthian State and then the powerful presence of the Sassanid Empire along with the expansion of territory and protection of borders against the invasion of the Eastern nations. Despite these reasons, there is little information on past events in the area in the historical and archaeological sources. Iranian Khorasan occupies a large part of the northeast and east of the country. Archaeological survey in the middle part of the Kopeh Dagh-Ala Dagh corridor in northern Khorasan has identified various sites from the Parthian and Sassanid eras. Among the identified ancient sites, sixteen architectural structures were identified in the form of a fort. Recognizing the differences and similarities between the architecture of the forts with each other and then comparing them with other similar examples inside and outside the borders of Iran, led to a better understanding of the fort-building styles and their grouping in this region. In this research, in addition to identifying forts and recognizing their architectural features, using remote sensing, ecological studies and the type distribution of forts from each other. This article emphasizes the socio-political role of the study area as the last border access to the central plateau of Iran.
Keywords: Northern Khorasan, Kopeh Dagh-Ala Dagh Corridor, Tappe-Ghaleh & Forts Parthian, Sassanid.

Introduction
To understand the cultural developments of the Parthian and Sassanid communities in the northern region of Khorasan, it is important to know the nature of the settlements of this period. In written sources, Greater Khorasan is often depicted on the northeastern edge of the civilized world. This region has always played a dynamic role in ancient times due to the function of the pathway and the connection of the East to the West in the field of population movement with the entry or invasion of nomadic tribes and cultural exchanges. The origin and emergence of the Parthian state, the development of eastern territories and the control of borders and communication routes, and the monitoring of trade were other reasons for the importance of this region during the Parthian and Sassanid eras. These subjects had caused the geographical-political-administrative boundaries of the two governments in Khorasan to be constantly changing.
Aims and Necessity of the Research: Typology of forts of Parthian and Sassanid historical periods in the middle part of Kopeh Dagh-Ala Dagh corridor is part of the purpose of writing this article, in addition to the identification of architectural styles and their comparison with other simultaneous examples in the territory of these two governments. The function of forts and their role in the socio-administrative structure presents their historical context and also highlights the importance of the studied geographical area in the political-governmental structure of the Parthians and Sassanids.
Research Questions: What are the architectural features of the forts during the Parthian and Sassanid eras? Is it possible to provide a relative chronology for forts based on architectural features? Did the form and shape of the forts affect their functional role? What was the position of the studied geographical area in the Parthian and Sassanid historical governmental structure?
The middle part of the Kopedagh-Aladagh corridor is located in the northeastern part of Iran and northern Khorasan and is based on the political divisions include the cities of Farooj and Shirvan in North Khorasan Province and the city of Quchan in Khorasan Razavi Province. 
The first systematic survey was conducted in 1977 in the Upper Atrak area (from the western part of Quchan to Shirvan). Archaeological studies have been re-conducted by the author in the last two decades in the middle part of the Kopeh Dagh-Ala Dagh corridor. These studies led to the identification of various sites from the Parthian and Sassnid periods. These studies led to the identification of various sites from the Parthian and Sassanid eras. Sixteen architectural structures were identified in the form of forts. Recognizing their architectural features, differences, and similarities with each other and then comparing them with other similar examples inside and outside the borders of Iran, led to a better understanding of forts construction styles and their categories in this region. By studying the characteristics of forts, they can be divided into two groups according to their geographical location. 1- Mountain fort: Two mountain forts are located on the northern and southern slopes of the Middle Mountain Valley. The position of these two sites is very significant because they had complete control over the opposite plain and the surrounding area. 2- Plain forts: As it is clear from the title, this group of forts is in the plain but based on their physical characteristics, they are divided into three subgroups: a) single forts. b) a fort with a non-enclosed settlement area. c) enclosed settlements. All the structures introduced in the three groups of plain forts, despite the differences, also have common points with each other, which indicate the existence of a common style of architecture and its continuation from the Parthian to Sassanid period, and the differences can indicate the different functions of these structures.
The eastern borders of the territory of the Parthian and Sassanid governments have been one of the main gateways for the invading tribes such as the Scythians, Khyuns, Heptalians, etc. These attacks often occurred from three entrances. 1- Merv route to Mashhad-Neishabour, 2- Herat route to Taybad-Neishabour, and the third route was crossing the northern margin of Kopeh Dagh and entering from Gorgan to the central plateau of Iran.  But in northern Khorasan, the Kopedagh mountain range, as a natural wall, has been part of this defensive system. The Kopeh Dagh-Ala Dagh corridor in the east-west direction has been a geographically enclosed and closed area between two wide areas, namely the Akhal plain in the north and the central plateau of Iran in the south. Therefore, due to this natural feature, it has played a deterrent role against the direct attack of the invading tribes on the central plateau of Iran.

Conclusion
By studying all the conditions affecting the subject of political-administrative, economic-social and military structures in the middle part of Kopeh Dagh-Ala Dagh corridor, it can be concluded that this region, in addition to having relative military security, economically meets the needs of the region’s inhabitants (although it was independent, but politically it had to be under the tutelage or part of one of the satraps of its time. Perhaps it can be argued that due to the density of Parthian and Sassanid castles and sites on the southern slope of Kopeh Dagh, especially their density in front of a passage in the north of Farooj city, was due to a political and administrative relationship with a city in the north of the region. According to archeological excavations and historical sources, the two important cities of Nisa and Merv from the Parthian and Sassanid periods in the north of this region have been the political center of the region. On the other hand, based on the interregional route, the northern and southern parts were interconnected, so the region may be politically subordinate to one of the two states of Parthia or Merv, depending on the time and government.