Originally a Roman fortress constructed under Emperor Caracalla to protect its inhabitants from Bedouin tribes, this site dates to the second and third century AD, although there is trace evidence of Nabatean presence at the site. It was one fort of many on the Roman highway, Via Nova Traiana, a route that connected Damascus to Aqaba by way of Petra and Amman. However, by the eighth century, the Umayyad Hisham ibn Abd al-Malik ordered for the Roman structures to be demolished in order to redevelop this military site and its neighboring territory to become one of the grandest of all Umayyad desert complexes.

http://en.wikipedia.org/wiki/Qasr_al_Hallabat