The earlier Sasanian rulers may have begun to erect fortifications against the Romans in Mesopotamia, and against nomadic enemies north of the Caucasus, but Shapur II extended the system of defence, probably in imitation of Diocletian's activities in building the limes of the Syrian and Mesopotamian frontiers of the Roman empire. Islamic writers attribute to Shapur II the line of forts, walls and probably moats or ditches situated in Iraq on the edge of the desert and called khandaq Sabur.
Just as the Romans settled limtanei on their frontiers, so Shapur settled Arabs in Iraq as a permanent defence force against other Arabs of the desert, especially those allied with Rome. We have no information about Shapur's efforts to repel invaders from the Caucasus region, but we may assume that the famous wall of Darband, if not begun by Shapur, at least was the result of his efforts at fortification in the north. The system of Roman limes must have impressed the Sasanians for it stopped Shapur's strenuous efforts to repeat what his great-grandfather Shapur I had done. And this was in spite of the fact that the army of Shapur II probably was better organized and more disciplined than under previous monarchs.
In addition to the limes and system of forts built by the Sasanians, mainly to halt raids by the desert Arabs, either brigands or allies of the Romans, there were a number of buffer states at the beginning of Sasanian rule. These became absorbed into the central state in the course of time, such that by the 7th century even the buffer state of the Arab Lakhmids of Hira was gone. The end of the buffer states may have weakened the Sasanian state at the time of the Arab conquests, for the imperial forces had to bear the brunt of the first attacks. In the 3rd century such client states as Adiabene, Arabistan (in the north-east Syrian desert), and of course the Lakhmids, played an important role in Sasanian-Roman relations, perhaps comparable to Palmyra on the Roman side.
Just as in the Roman empire, the client states were taken over by the central government, and in their place the fortified cities of the frontier and the limes were organized into a defence system. In the long series of wars between the Sasanians on one side, and the Romans followed by the Byzantines on the other, the frontier remained more or less constant in upper Mesopotamia. It is true that sometimes Nisibis, Singara, Dara and other cities of upper Mesopotamia changed hands, but the stability of the frontier over centuries is remarkable. Although the possession of frontier cities gave one empire a trade advantage over the other, one has the impression that the blood spilled in the warfare between the two states brought as little real gain to one side or the other as the few metres of land gained at terrible cost in the trench warfare of the First World War.