Ruins of the medieval town-fortress Hotalich are located 4km north-west from the today`s town of Sevlievo. The fortress was built by the Byzantines in the 5th century. In 10th century, at the same location, the Bulgarian state began construction of defense facilities to defend the country from the barbarian invasions from the north and Byzantium from the south.
The fortress was the largest in area except these of the old Bulgarian capitals Pliska, Preslav and Veliko Tarnovo.The excavations of medieval town of Hotalich and the fortress began for the first time in 1981. The archaeologists were found a high tower, two parallel walls (first of them dates back to the Early Byzantine period and the second from the 9th-10th century), remains of two main gates, ruins of quarters, churches, palace of the boyar (the local ruler), etc.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.