the great thing about this City is the fact that it survived about 2500 years inhabited and almost intact.the Romans, Nabateans, Byzanthine, Umayyad Muslims all left traces in it..it is simply an open museum. make sure to read about it before visiting or at least get a guide book to the numerous sites you are about to see or you will be lost and not really appreciate what you are seeing.
you might wanna know how important it is to the Eastern Christians since it was a center of archbichop and it has many intact ancient churches. alo very important for muslims since in it prophet Mohammed has met the monk Buheira who was the first to forsee the child Mohammad's prophesy. It was also the second Nabatean capital after the sacking of Petra by the Romans.
truely a must see if you are spending longer than a few days in Syria.the area also has many other ancient classic cities like Shahba, Qanata(Qanawat),Izraa, Salkhad..etc