In the 6th century, Nikopolis ad Istrum was rebuilt as a powerful fortress enclosing little more than military buildings and churches. The largest area of the extensive ruins (21.55hectares) of the classical Nicopolis was not reoccupied since the fort covered only one fourth of it (5.75 hectares), in the southeastern corner. The town became an episcopal center during the early Byzantine period. It was finally destroyed by the Avar invasions at the end of the 6th century. The town was mentioned, at that period, by the Byzantine historian Theophylaktos Simokates in connection with the campaign of the Eastern Roman military commander Komentiolos against the Avars in 598. A Bulgarian medieval settlement arose upon its ruins later (10th-14th century). The Turkish name of the town was Eski Nikup, which means "Old Nikopolis".
http://en.wikipedia.org/wiki/Nicopolis_ad_Istrum
http://www2.rgzm.de/Transformation/Poland/StrPln21EN.htm