Abstract
Tol-e-Zahak site, as the ancient city of Fasa in the historical and Islamic period, is located about 2 km south of the modern city of Fasa in the eastern center of Fars province and was registered as the 15th national monument in 1310 AD. After conducting a archaeological systematic survey of this site in 1397, in addition to identifying the extent of the site (based on the dispersion of surface data) which reaches more than 500 hectares; Chronology and settlement periods of the site, which indicate its habitation from the late fourth millennium BC to the Islamic Middle Ages; It was found. By studying the data obtained from the survey, especially pottery samples, one of the important settlement periods of Tol-e- Zahak area is the settlement related to the Sassanid period. Scattering of cultural materials related to this period (especially pottery) was observed almost all over the site. In this article, the authors intend to examine the historical sources and previous archaeological research in the region in connection with the Sassanid period in the region, the results with the data related to the Sassanid settlement identified from the methodical study of Tol-e- Zahak site, Intertwined (Historical Approach) and to reconstruct the appearance of Fasa city in the Sassanid period as much as possible. Studies show that the Sassanid settlement of Tol-e- Zahak site is undoubtedly a large urban settlement of the type of cities with an irregular shape of the Sassanid period. Due to its strategic location and suitable environmental conditions for agricultural and livestock activities, this city as one of the important commercial and commercial cities throughout the Sassanid rule and even a few centuries after the advent of Islam in Darabgerd province in the east of Persia Has played a role.
Keywords: Tol-e- Zahak Site, Ancient City of Fasa, Archaeological Survey, Sasanian Period.

Introduction
Fasa is a city in the eastern central of Fars province, despite the environmental conditions suitable especially for agricultural activities and also its special strategy position, which is located between the civilization areas of the east and west of the Iranian plateau on the one hand and the access route to the open waters of the Pars sea, despite the identification of many works and sites from prehistory to late centuries, Islamic archeological knowledge, especially the situation of the region in its historical period is very small Zahak bridge (Tol) is the largest ancient site of the city and one of the most important ancient sites of Fars. Most of the researchers in the area are related to the pre-revolutionary period. Some limited investigations were carried out on each of these sites based on a specific settlement or only included the main mount of the area which gave its name to the entire part. So it’s necessary to carry out a systematic investigation that covers the entire area of the site and all settlement periods. After the systematic investigation of the site that was carried out in 2017C (1397) based on the distribution of cultural materials. 
The surface area of the site was more than 500 hectares it was reached with the chronology of the cultural materials of the period of the residence in the site from the end of the fourth millennium to the middle Islamic century it was estimated continuously without interruption it seems that despite the most Sasanian cities during the Sasani’s period, the city of Fasa has been an important commercial city in the region since and it reached the peak of its expansion and prosperity in the Sasanian and early Islamic period. On the other hand, the historical and geographical sources of the beginning of Islam have a close relationship with Sasanians and called the city a large city with many products and prosperous commerce a city that has all the important structural features like; Kohandej, Baroo, Khandagh Sharrstan, Rabz… .

Data
The largest number of sites identified in the archaeological survey of Fasa which was conducted in 2008 (1388) belongs to the Sasanian period. A total of mounds and artifacts have been identified from this period (image 3). Of these 18 sites, only Sasanian settlements Also 28 sites were identified with the continuity of settlement from the Ashkanian to Sasanian period, 1391(166), the results of archaeological investigation and excavations region in Fasa indicate the importance of the region such a way that all the boundaries of the city witness settlements related to this period include urban and rural settlements, (watch towers), (fire temples), (water structures), (cemeteries), etc.
The main part of this article is derived from the results of the methodical archaeological survey of the site of Tol Zahak, it’s necessary to give a brief explanation of the survey and sampling method chosen for this site. Since the area in question is located between the modern city of Fasa and the two villages of Dastjeh and KheyrAbad, most of its lands has been cultivated and turned into agricultural lands for the villagers on the other hand the urban and rural construction in the northern, southern and eastern parts of the are an don’t allow a more accurate estimate of the area out to the size and extent of the area the selection of small square brought problems. So it was decided the entire area can be gridded into units with dimensions of 100 × 100 meters in the main geographical directions. Sampling to select the sample units in this research is based on one of the pre-defined, scientific methods, which is stratified methodical sampling.

Discussion
General chronology of Tol e Zahak and recognition of the settlement periods as well as identification of each period is based on the pottery chronology obtained from the methodical investigation of this area. According to the existence of prominent Sasanian cities explored near Fasa such as Darabgerd, Ardeshir Khoreh, Fasr AbuNasr, etc. Knowing the Sasanian pottery of this area is not very complicated. They have enough cooking. Pottery with a thick red mud coating and gray pottery with a black coating (image 6) of the common types of Sasanian pottery of the Tol-e-Zahak area which were identified from almost all investigated units. Among the interesting points of Sasania’s pottery of the Tol-e- Zahak is the use of decoration. There is a large undulating log on their outer surface. According to the distribution of Sasanian pottery in the Tol site, the Sasanian settlement covers the entire surface of the site, and it can be called a large urban settlement that is located next to the important cities of Kureh. Darabgerd has been in the eastern center of the Pars state based on the large dispersion of pottery known as Sasanian-Islamic pottery on the entire surface Fasa has passed the transition from the Sasanian period.
In the Islamic period without any change in the urban structure of the area of Tol -e-Zahak, several types of pottery related to this period were found. One of these characteristic types is pottery with an alkaline green glaze with added motifs which was obtained from most of the sampled units of Tol Zahak (image7) other types of Pottery related to this period are pottery with carvings decoration with a wave on the edge and other surface and the pottery is decorated with a grooved log pattern and these samples are almost was found all over the area. Sir Orel Eshtein the first explorer of Tol Zahak describes it as follows: there isn’t a big ancient area in Fasa except Tol Zahak so without doubt this area is the old city of Fasa (eshtein,1936) but this hypothesis was scientifically proven by conducting a systematic investigation. Conditions suitable environment and special strategic location of Fasa due to its location between several royal cities.

Conclusion
Fasa City has different from other important cities of the Fars State in the Sasanian period such as Bishapor, Darabgerd, ArdeshirKhore, and, Estakhr the most important issue is the function of the city and its background and history. The most important cities in Fars in the Sasanian period can be considered royal and religious cities that played a big role as religious centers of the Sasanian period. Without any doubt, the existence of significant governmental and religious structures in cities is undeniable but Fasa can only consider the commercial prosperity of commerce and agriculture as the most important factors of the city’s prosperity in the Sasanian period. It is the commercial center of Fars State and most of the geographers who have visited this city are described in their works. Trade and commerce of Fasa have been praised a lot. Archaeological evidence shows that the Sasanian-Islamic period of Tol Zahak is one of the large and important settlements of the site the Sasanian period and the early Islamic centuries can be concluded this transfer of power from Sasanian to Muslim Arabs which includes the aspect political and religious issues couldn’t have created a disturbance in the prosperity of Fasa.
Their fore description of the large size of the city included commerce and agriculture can imagine the Sasania’s area well. According to the description of the surface, Fasa is estimated to be about 500 hectares and is one of the largest cities. Another issue is related to the historical region in the Sasanian cities. Most of the royal cities in Fars as BiShapor were founded in this period and Darabgerd and Ardeshir Khoreh a little time passed it was used like checkered entities but about the formation of city it is very different issue.