The defense system of Byzantium on the Balkan Peninsula involved three main fortified lines: the Danube Limes, the Balkan Range and Strandzha Mountain. It was finally accomplished in the course of the 5th – 6th c. Fortresses great in number protected the area enclosed between the Danube River and the Balkan Range. For most part they followed the course of the rivers flowing there, Vit River including. The distances between the fortresses was usually 5 to 10 km as often two neighbouring ones enjoyed a direct visual connection. Many pairs of fortifications are now known set up on both sides of a river gorge – both fortresses by Sadovec are a rather informative example.