The first fortress walls of Nessebar were built by the Thracians in the 8th century BCE. With the time, after the arrival of the first Dorian settlers at the end of the 6th century BCE, the town grew significantly and the need for stronger walls appeared. In the the 5th century BCE, the fortification walls were expanded. Romans also strengthened the walls. Nowadays, the best preserved ruins are from the 5th century ACE, during the rule of Byzantium.