From the mid seventh century to the beginning of the ninth, the construction of fortifications (josaku) in ancient Tohoku concentrated in four periods, constituting points of change in policy regarding the region. First, the Nutari and Iwafuku fortifications were related to the northern expedition of Abe no Omi, and it is presumed they were intended to serve the trade with the north and as a port of entry for the continent. The Koriyama phase II government office of the end of the seventh century was built as a central administrative facility for control of the region.<BR>In the second stage, around the years 720 to 730, the castles of Tagajo and Akitajo were built as provincial government facilities for Mutsu and Dewa, while the so-called " five fortresses of Tenpyo" were erected on the Ozaki plain. This concentration of fortifications on the Ozaki plain formed a line of defense on the northern perimeter of the area for which the system of administrative districts (kori) had been implemented, and was intended to protect and assist settlers in this area, and to serve as a defense against conflict anticipated between them and the indigenous Ifu. In this manner a meticulous program of dual control was carried out by combining fortifications for dealing with the Ifu with administrative offices for dealing with citizens proper (komin).<BR>In the third stage Momo'ojo, Ogachijo, and Ijijo fortifications were constructed around 760 for the purpose of establishing administrative districts and for the direct supervision and protection of the settlers. Repairs were also undertaken at this same time at Tagajo and Akitajo.<BR>The fourth stage was around the year 800, when a large-scale reorganization was carried out with the construction of the new fortifications of Isawajo, Shiwajo, Hotta, and Kinowa, through renovations of the provincial government facilities of Mutsu and Dewa, and by closing down old fortifications such as Momo'ojo and Ijijo. These changes stemmed from the policy of territorial expansion of the Enryaku era, the most aggressive period in the administration of the Tohoku region.<BR>Through subsequent administrative reforms, spurred by the debate regarding benevolent government, facilities were concentrated from the mid ninth century on at the six fortifications of Tagajo, Miyazawa (Tamatsukuri no ki), Isawajo, Kinowa (which became the provincial government facility for Dewa), Hotta (the successor to Ogachijo), and Akitaio.<BR>This system of six fortifications had as one of its purposes the division of responsibility for dealing with the various problems of each geographic area, while it also played a large role in the direct administration over wide areas in the north by provincial governments, in lieu of a system of district administrative centers. This was the result of the abandonment of the program to establish administrative districts under the Ritsuryo system, as the aggressive military and settlement policies of the central government changed and were forced to be abolished, and remained the basic administrative structure for the Tohoku region until the decline of the fortifications in the middle part of the tenth century.