During the centuries a lot of fortified systems have been built which made of the peninsula an impregnable fortress.
The first inhabitants on the peninsula – the Thracians, started the fortification of the settlement in the 8th century BC. A fortified wall with a gate has been found from this period. It was made of hewn stone joined by mud. The wall that can be seen nowadays is 2 m high.
During the time of Hellenistic colonization (6th century BC) the colonists from Megara did not start immediately the construction of a new defense system. They built a new wall later, in the end of the 5th century and the beginning of the 4th century BC. The work lasted till the 3rd century BC. Big stone blocks were used, their face sides being specially polished, placed in line, without any solder. The inner part of the wall was felled with broken stones and soil. The Greek wall was well studied in the west part of the town. It can be seen that in some places it overlaps the later one – the Byzantine. Messambria being conquered by the Romans, the found defense system was preserved and used during the following centuries.
After the decay of the Roman Empire the Byzantine rulers undertook an active constructional work and in the middle of the 5th century AD a new fortified wall has been built in “opus mixtum” – alternation of stones and bricks. The best-preserved part of it can be seen today near the city gate, opposite to the strip. Here the wall was faced with big orthogonal hewn stone blocks. The gate was flanked with two square towers on both sides of which round and semicircular towers were symmetrically placed. Two gates – a drawbridge like one and a twin leafed gate closed tightly the access to the town. During the Early Byzantine period a fortified wall was built on the south – west coast, near the port.
The early Byzantine system has been used during all the Middle ages. In the time of war attacks of the enemy part of it was destroyed seriously but it was quickly rebuilt. The first reconstruction was dated to the end of 6th century and the beginning of the 7th century, after the attacks of the Avars. After khan Kroum conquered the city in the year 812 he pulled down the wall. Later, in the end of the century, it was rebuilt of bricks. In the end of the 11th century the defense system was repaired once again. During the reign of Tsar Ivan Alexander the walls were definitely reconstructed.
After the town was subjected to Ottoman rule in 1453, the defense system lost its strategic importance and, left without any special cares, gradually declined.