Coins from Eastern Celts
Gallic groups, originating from the various La Tène chiefdoms, began a southeastern movement into the Balkan peninsula from the 4th century BC. Although Celtic settlements were concentrated in the western half of the Carpathian Basin, there were notable incursions, and settlements, within the Balkan Peninsula itself.

From their new bases in northern Illyria and Pannonia, the Gallic invasions climaxed in the early 3rd century BC, with the invasion of Greece. The 279 BC invasion of Greece proper was preceded by a series of other military campaigns waged toward the southern Balkans and against the kingdom of Macedonia.

A part of the invasion crossed over to Anatolia and eventually settled in the area that came to be named after them, Galatia.
Eastern Celts
The Scordisci were a Celtic Iron Age tribe centered in the territory of present-day Serbia, at the confluence of the Savus, Dravus and Danube rivers. They were historically notable from the beginning of the third century BC until the turn of the common era.

They consolidated into a tribal state. At their zenith, their core territory stretched over regions comprising parts of present-day...
The Serdi were a Celtic tribe inhabiting Thrace. They were located around Serdika, now Sofia in Bulgaria, which reflects their ethnonym.

They would have established themselves in this area during the Celtic migrations at the end of the 4th century BC, though there is no evidence for their existence before the 1st century BC. Serdi are among traditional tribal names reported into the Rom...