byzantine fortress ruins of the ancient thracian settlement mesembria. the old town of nessebar on the black sea coast, unesco

fortress ruins nessebar ancient mesembria fort old nesebar town bulgaria heritage unesco byzantine thracian antiquity architecture attraction balkans battlement building bulgarian castle city construction culture europe exterior historical journey sea black tourism travel trip destination burgas settlement defense island coast walls antique landmark stone embankment famous fortification masonry archaeological citadel mesembria world region