Fortresses towns and garrisons of the eastern frontier: Zenobia, Sergiopolis, Qasr ibn-Wardan
  The Early Byzantine period saw the reintroduction in the Mediterranean of the walled city. Walls contributed to the recovery of urban life from the third-century crisis, and to the struggle to survive further threats. The fortifications of the cities on the eastern frontier were thus a focus for Justinian's construction activity. Zenobia-Halabiye, on the Euphrates, was enlarged and fortified by two of Justinian's mechanikoi (architects having a ground in mathematics), John of Constantinople and Isidore the Younger. The latter was the nephew of Isidore of Tralles, the architect of St Sophia, and was also appointed to supervise the reconstruction of St Sophia after its original dome collapsed in 558. Zenobia's massive walls rise steeply to an unassailable citadel; they comprise a three-storey guardhouse (praetorium) and are protected from the rising waters of the Euphrates by a breakwater.
  In 527-542, the emperor gave huge fortified walls, as well as colonnaded streets, cisterns, baths and churches, to the remote pilgrimage centre of Resafa-Sergiopolis (named after the venerated saint, Sergius). The walls enclose an area of roughly 540 by 380 m and are equipped with square, polygonal and round towers nearly 12 m high. Of the four gates of the city, the one facing the pacified parts of the empire to the north was decorated with half columns and blind arcades, all richly decorated. An elongated tetraconch and Basilica B, now identified as the shrine of Sts Sergius and Bacchus, were probably erected as part of the imperial building program. The former lies close to the north gate, and was the first to be approached by incoming visitors as they proceeded along the colonnaded street towards the southern sector.
  Basilica B has an apsed sanctuary flanked to the north by a triconch martyrial chapel. The triconch was lavishly decorated with floors of opus sectile,painted carved ornament and gold mosaic. A reliquary sarcophagus stood in the eastern conch. A square chamber was added later next to the triconch in order to house a second sarcophagus, which was provided with a rectangular cavity into which the pilgrims poured libations. The two reliquaries may have commemorated the local saints Sergius and Bacchus. Justinian and Theodora are known to have presented the shrine with a gemmed cross, which was seized in 540 by the Sassanid king Chosroes I, together with the gold revetment of the saint's tomb and other treasures. In 591-592, as a thanksgiving to St Sergius for a military victory and the birth of a son, Chosroes II returned Justinian's cross and gave the shrine several gold votive objects. A second basilica (Basilica A), identified by an inscription as the Church of the Holy Cross, was built in 559 by bishop Abraham. It was linked to a large ecclesiastical establishment, monastery, or pilgrim's hostel, and displays the typical Syrian bema in the middle of the nave.
  The importation of Justinianic architecture in Syria is best illustrated by the military outpost of Qasr ibn-Wardan (561-564), and its use of brick and domed structures. The complex, which was probably the residence of a military commander, comprises a palace, a church and a barracks building. Brick, a material typically associated with the Aegean coastlands and Italy, was used throughout, alternating with bands of local basalt (brick was also used for the cisterns of Resafa, and the walls of Zenobia). Domes, a clearly Constantinopolitan feature, covered the palace audience hall and the church.