In the early period of the Sasanian Empire, a number of buffer states existed between Persia and the Roman Empire, which played major role in Roman-Persian relations. Both empires gradually absorbed these states, and 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,[6] Nisibis, Amida, Singara, Hatra, Edessa, Bezabde, Circesium, Rhesaina (Theodosiopolis), Sergiopolis (Resafa), Callinicum (Raqqa), Dura-Europos, Zenobia (Halabiye), Sura, Theodosiopolis (Erzurum),[7] Sisauranon, etc.