Structure of defense fortifications and their roles in providing security in Khorasan and Transoxian during the early centuries to the Middle Ages

Defense-security structures in the history of architecture of the Islamic Iranian cities are powerful and magnificent elements that showed themselves when entering the cities.  What physical position and what role did these structures have in the cities of Khorasan and Transoxiana until the Middle Ages?  This question forms the center of the present historical research which is conducted by descriptive-analytical method and library collection.  Findings show that the location of the physical system of defense fortifications is derived from the general structure of the cities of this period that each city had three layers (Rabz, Sharestan and Arg) with different functions and separated from the center to the periphery and from the suburbs to the center. Defensive fortifications were the outermost physical structures of all three layers, including walls, towers, gates, and ditch s.  The wall was the basis for the construction and determination of the physical shape of the other elements (tower, gate and ditch ).  Due to its large population, extensive area  and supply of economy,and its location which as the most external region encompassed Sharestan and Arg, the area of Rabz as the most important security sector had a deep ditch , in addition to a wide and long wall around the wall or communication part (gates). The invaders first had to cross the defensive fortifications of Rabz and also cross the fortifications of Sharestan to enter into this area, to finally capture the administrative and command section of the city, which was the smallest part of the city.