In Late Antiquity, one part (about a sixth) of the area surrounded by the Augustan city-wall was enclosed by the new wall. This fact means not only that the population of the late antique Nikopolis was probably diminished, but also that Nikopolis was a local strategic centre and the city worth defending by constructing the massive walls.
The late antique wall was first constructed in the fifth century, and in the next century it was repaired by Justinian.
In the photo above is the West Gate. Here you see only one tower, but there is another one to the right, behind the trees. These huge horseshoe-shaped towers are often compared with the similar ones in Constantinople.