The modern capital city of Bulgaria, Sofia, is located in the Balkan Peninsula, on the foundations of Roman Sardica or Serdica, a name that derives from the Thracian population of Sardj or Serdj. The name Sardj was borrowed from one of the Celtic tribes with which the Thracians coexisted. After the 4th-century BCE Macedonian occupation, the Sardj were conquered by the Romans in the third decade of th…