This is one of the most impressive military defence facilities from Bulgaria’s antiquity and Middle Ages. It is built by the Emperors Diocletian and Constantine the Great in the fourth century and restored under the Emperor Justinian the Great (527-565 years) as an unmatched fortification in late antiquity. This is a strong polygonal fortress, measuring 350 x 240 meters, with walls thickness from 3.60 to 4.20 meters, with two gates and more than 12 m height pentagonal and triangular towers. This fortress destroyed by Slavs and Bulgarians in the seventh century, is recovered from Khan Omurtag (814-831 years) and used as a fortress city, without interruption, until the nineteenth century.