complex torsion equipment was rare, since traditional Persian expertise in archery reduced their apparent benefits. Elephants were employed (e.g. as siege towers) where the terrain was unfavorable for machines. Recent assessments comparing the Sasanians and Parthians have reaffirmed the superiority of Sasanian siegecraft, military engineering, and organization, as well as ability to build defensive works. By the beginning of Sasanian rule, a number of buffer states existed between the empires. These were absorbed by the central state over time, and by the 7th century the last buffer state, the Arab Lakhmids, was annexed to the Sasanian Empire. Frye notes that in the 3rd century AD such client states played an important role in Roman–Sasanian relations, but both empires gradually replaced them by an organized defense system run by the central government and based on a line of fortifications (the limes) and the fortified frontier cities, such as Dara. Towards the end of the 1st century AD, Rome organized the protection of its eastern frontiers through the limes system, which lasted until the Muslim conquests of the 7th century after improvements by Diocletian. Like the Romans, the Sasanians constructed defensive walls opposite the territory of their opponents. According to R. N. Frye, it was under Shapur II that the Persian system was extended, probably in imitation of Diocletian's construction of the limes of the Syrian and Mesopotamian frontiers of the Roman Empire. The Roman and Persian border units were known as limitanei and marzobans, respectively. The Sasanians, and to a lesser extent the Parthians, practiced mass deportations to new cities as a tool of policy, not just the prisoners-of-war (such as those of the Battle of Edessa), but also the cities they captured, such as the deportation of the Antioch's people to Weh Antiok Khosrow, which led to the decline of the former. These deportations also initiated the spread of Christianity in Persia. The Persians seem to have been reluctant to resort to naval action. There was some minor Sasanian naval action in 620–23, and the only major Byzantine navy's action was during the Siege of Constantinople (626). Assessments The Roman–Persian Wars have been characterized as "futile" and too "depressing and tedious to contemplate". Prophetically, Cassius Dio noted their "never-ending cycle of armed confrontations" and observed that "it is shown by the facts themselves that [Severus'] conquest has been a source of constant wars and great expense to us. For it