During centuries, fortification walls and towers were built to protect ancient Nessebar. The first inhabitants on the peninsula were the Thracians who started to build these walls in the 8th century BC. From this period, a fortified wall and gate were found and can be seen today. After the decay of the Roman Empire, Byzantine rulers started an active construction work in the middle of the 5th century AD. The built walls were in style opus mixtum- an alternation of bricks and stone. Best preserved walls can be seen today near the city gate. The early Byzantine construction system was used in the middle ages as well.
Due to enemy war attacks, part of the walls were destroyed, but then quickly rebuilt. After the Ottoman rule in 1453, the fortification walls lost their significance and strategic importance and were left in ruins.
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.