Abstract
Despite the importance of the eastern regions of Iran, especially South Khorasan in the process of historical developments in Iran, archaeological research has not been done in this region. Kahnak archaeological site is an important Islamic site in South Khorasan province in the east of Iran where the weakness of environmental conditions and climatic constraints, especially the lack of water resources, has led to the formation of large and densely populated areas and settlements in this region. As this site provides economic, social, and cultural connections between the northern (Khorasan) and the southern regions (Sistan), which is also on another main route, it is very important for archaeological studies in the east of the country. Main routes have been a decisive factor in the location of human groups throughout history, and this has been more influential in areas where their habitats have been adjacent to major trade routes and communication roads. Kahnek site is considered as a place for educational activities of the Department of Archeology of Birjand University. From 2014 to 2019 excavation activities in this area have continued for six consecutive seasons. In this research, the fourth season of these excavations and their findings are introduced. The study aims to identify and analyze the architectural space called Kahnek Castle on the east side of the area in the winter of 2016. According to the surface data and excavations, the Kahnek site belongs to the third and fourth centuries until the Timurid period, and most of the gathered data include potteries and architectural structures are related to the Islamic Middle Ages. The architectural structures found in this season of excavation are mainly related to the architectural spaces of the castle. The present work is carried out in descriptive-analytical method and historical-cultural approach based on field data and library resources. 
Keywords: Establishment of the Islamic Period, Archaeological Excavation, Sarbisheh City, Kahnek Site.

Introduction
Regarding the archeology of the east of the country, especially South Khorasan, no studies have been conducted in this region, and the knowledge of researchers from the mentioned areas is more limited to the information contained in historical sources and texts and so far few articles and books about Archeology of this area has been published. One of the important sites of the Islamic Middle Ages in this region is the Kahnek site in Sarbisheh city. This site was selected in 2014 by the General Directorate of Cultural Heritage, Tourism and Handicrafts of South Khorasan and in collaboration with Birjand University as an internship site for archeology students of the Faculty of Arts - Birjand University and archaeological activities continuously until 2019, In this, it has continued. In the past, the Kahnek area was located as a settlement on an important communication route, which according to the cultural data obtained, it was determined that the establishment of Kahanek continued its life from the third and fourth centuries AH to the late Islamic (Safavid) centuries. In this study, the findings of the fourth chapter of the site are introduced. The cultural data obtained from the excavation of this site, in addition to architecture and pottery, have been other materials such as animal bone remains, decorative and functional metals, glassware pieces, metal and pottery ornaments, etc. This research tries to address these questions: 1- What is the plan of the explored architectural spaces of the ancient castle? 2- Can the influence of neighboring cultures be seen in the pottery of the Kahnek area? The research hypotheses are as follows: 1- According to the excavations, the old castle has a relatively circular plan and limited architectural spaces with square rooms. 2- Kahnek site is affected by neighboring areas due to its location in an important communication route that extends from north to south so that Neishabour-style pottery is one of the leading pottery styles of this area in the first centuries.

Geographical Location and Description of Excavation in Kahnek Site
Kahnek area is located in South Khorasan province, Sarbisheh city, in Mo’menabad rural district of the central part of this city and 50 meters east of Kahanek village and is located at 764602 longitudes and 3603433 latitudes. Architectural spaces in the fourth season of excavating the old area in 2015 are related to rooms with dimensions of 2×3 meters, which are related to the space of the castle tower. The potteries obtained in this excavation season are divided into two glazed and unglazed groups belonging to the early to late Islamic centuries. In addition to the potteries, an oil lamp, a metal scrap (possibly a coin), parts of quern-stones, a saw, pieces of glass, a chunk of glazed brick, a sharpening knife, and remnants of animals bones were found. According to the excavations and cultural materials and architectural evidence, three stages can be considered for this site. In the first and new stage of the site, it has been used as a temporary settlement in connection with livestock and agriculture. The second stage is related to the Islamic Middle Ages and at the same time with the Seljuk, Ilkhanid, and Timurid eras in which the site is at the greatest prosperity and development. The third stage of the area and the oldest one is related to the early Islamic centuries and the formation of the area, which shows the extensive connections of this area with northern Khorasan. At this stage, the architectural structures are built of clay with a wide foundation and thinner upperparts.

Conclusion
Archaeological excavation of the Kahnek site was carried out in 2016 with the cooperation of the General Directorate of Cultural Heritage, Tourism and Handicrafts of South Khorasan and Birjand University. The findings in this season of the excavation are the architectural works of several interconnected rooms with different dimensions. The walls of these architectural spaces are mostly made of clay; adobe and stone are rarely used in their foundations. Since most of the architectural spaces are detrital and in some spaces the adobes are placed diagonally next to each other, the roof of the residential spaces was probably made of rough-arch adobe. The pottery samples of the Kahnak area, which are the most important documents for the chronology of this area, can be compared with the samples and potteries of different regions in South Khorasan, particularly in North Khorasan and Kerman. The results of studies of samples and their comparisons show that the pottery in this area is related to the early Islamic centuries to the late Timurid and early Safavid periods. It can be said that trade routes played a key role in the process of communication and cultural similarities between different regions. Moreover, the Greater Khorasan Road is not only important for east-west connections but also it has been important for connecting the north and south of the Iranian plateau. Main routes have been an important factor in the location of human groups throughout history, which is more effective in areas where their ecosystems are adjacent to major trade routes and communication roads. However, it can be said that the climate has the most important impact on the process of leaving the inhabitants of this area, and agriculture, which is the main economic pillar of the region, has caused it to be abandoned due to climate change.