It is clear from the royal correspondence of the Assyrian empire and from the annals of Assyrian kings that the construction of forts was an integral part of the permanent establishment of Assyrian sovereignty in newly conquered regions. Forts served as garrison outposts in formerly hostile areas and were therefore the first footholds of Assyrian expansion into recently annexed territories. They were military centres, from which campaigns and intelligence operations were conducted into and beyond the frontier, administrative centres where the daily affairs of the surrounding areas were directed and monitored, and communications hubs through which news and information were channelled. In addition to these roles, forts or garrison centres also served as conduits through which the Assyrian ideology of imperialism could be diffused into the periphery of the empire and the process of the acculturation of the “foreign” inhabitants of peripheral zones could be conducted.
Liverani has suggested that Assyrian military expansion was not a process of conquering contiguous areas, in which a clear line could be drawn between regions under Assyrian control and those that were not. Instead, the process of Assyrian imperialism was one in which “islands” of Assyrian occupation were planted in peripheral zones soon after military incursions. In regions of Assyrian expansion, “the empire was not a spread of land but a network of communications” between Assyrian strongholds. The spaces between the “islands” of this “network empire” were slowly filled in through successive military incursion. Following these conquests new forts or garrison towns were constructed at critical junctures, to protect and fortify the networks connecting the existing Assyrian strongholds, and foreign populations were forcibly settled in the surrounding countryside. Peripheral regions were not, therefore, brought under the Assyrian yoke solely through swift military action but by the gradual growth and spread of “islands” of occupation into new regions. These “islands” must have initially consisted of forts or fortified settlements such as those referred to in the royal correspondence as birtu or HAL.ṢU meaning “fort”. This system of planting Assyrian garrisons in newly conquered regions is perhaps best exemplified in Nimrud Letter 48, in which the author speaks of “establishing the foundations (of a fort)” at several junctures in his campaign in the Iranian Zagros. As the system of Assyrian strongholds became more contiguous across the landscape, the area came more firmly into the grip of the imperial administration and the stage was set for expansion further into the periphery or into regions between these pockets of Assyrian control.