In the old part of the town of Sozopol is still preserved the south castle wall of the exisiting on this place ancient town of Apolonia. The castle was build in the middle of 5 century and is made of limestone. The walls are destroyed when the town is conquered by the Romans. A little after they are fully restored by local people.
The castle wall is discovered by accident while building a house on the same place. Besides the wall there are discovered a defence tower, a cereals barn, a well and part of the ancient system of drains. The barn is dated from 5-6 century, when the town of Sozopol was the biggest harbour for exporting cereals on the Black Sea. In the barn are found 12 big earthen jars and other smaller jars. It was used for about a thousand years. There were barracks over the barn, where the soldiers were living. The discovered ancient well is dated from 4-3 century b. c. It was part of a religious complex in honour of "The three nymphs". The system of drains in the ancient town of Apolonia was built in 5 century b. c. This shows the high level of the development of the ancient civilizations on these lands.