The Umayyad Mosque of Damascus is the earliest Islamic monument still extant after the Dome of the Rock in Jerusalem. As with the Dome of the Rock, the Mosque boasts a large volume of mosaic decoration of the highest quality that is believed to have been carried out by Byzantine artisans given the similarities of the technique and motifs with high-quality Byzantine commissions of the same era. The most notable difference is that there is a complete absence of figural imagery in the Islamic monuments. In the case of the Umayyad Mosque the decoration is particularly intriguing as it depicts a range of landscapes both urban and pastoral, all entirely without living creatures. This has led many commentators to argue that it represents a vision of paradise, with others arguing instead for an idealised representation of Damascus. Whether or not these interpretations are correct, the mosaicists appear familiar with Roman architecture, with porticoed late Roman villas appearing prominently in the decoration, meaning that the mosaics demonstrate a continuity with earlier artistic forms rather than a definitive break with the past. In many ways the decorative scheme is far more conservative than that of the Dome of the Rock, which predates is by over twenty years.
The impressively preserved city walls of Resafa are the subject of some debate when regards to their age and to who's reign their construction can be credited to. Scholars seem to be split as to whether they were constructed in the reigns of the Emperor Anastasius (491-518 AD) or the Emperor Justinian (527-565 AD). Procopius' attributes the first stone wall to the reign of Justinian. However this cannot be wholly accepted as fact as Procopius' accounts are occasionally deliberately misleading and sometimes wholly inaccurate. The most well preserved and impressive of the gates still extant is the Sura Gate on the north side of the city.
Regardless of which reign they were constructed in the defences do seem to be Late Antique. The walls, their covered galleries, the towers and gates were well preserved when I visited in 2010. As a result of the civil war their current condition is hard to ascertain.