Among the most significant constructions built in the first centuries of the Roman dominion was the fortified wall. It was finished around 176 - 180 A.D. during the co - rule of Marcus Aurelius and Lucius Aurelius Commodus Antoninus. The wall was destroyed not until the Ottoman conquest. The space inside the walls wasn't enough at some point and many buildings were built outside them. The form of the walls is rectangle and lengthened along the north - south axe with digression from the cardinal points to the northeast and cut northwest corner. There are different hypothesis about the reason for this cut out of the traditional rectangle roman settlements - presence of a river or already existing Thracian urbanization. From East the city is restrained from high geological terrace (now Alexander Battenberg square).
This is how the borders of the wall are set - 164 000 sq.
The main gateways are at the three walls where the exits of the main streets are. Exception is the Northern gateway which is where the northern and the northwestern walls gather. The first wall is built of bricks on top of a double - breasted plinth made from big stone blocks with 2 meters high foundations. The height of the wall is 8 - 9 meters and 2.20 meters thick. At around 50 - 60 meters distance along the wall are constructed round turrets, 2 - 3 meters above the wall itself.
The remains of this first wall are covered by the new one built at its place in the 4th century. It is made of mixed masonry (opus mixtum) - one after other layers of boulder and 4 brick rows. It follows the outline of the old one as somewhere the stone plinth is preserved. The plinth is 2 m thick. The round turrets and gateways are retained and their inner side doubles the wall by stairs. At some places there are side exits (poternae).
During the reign of Byzantine Emperor Justinian I is undertaken fortification of the existing wall surrounding it outside by new, higher wall thick 1.8 meters, built by brick on top of double - breasted stone plinth. At that time are brought in the triangle turrets set in the middle between the round ones. The turrets at the gateways are pentagon. It is made one more defend barrier - the so called proteyhizma (front wall) - 20 m ahead the fortress, 2 m thick with stone foundations and brick construction. Part of it with two triangle bastions can be seen under Knyaz Al. Dondukov Blvd. where the Eastern Gateway is.