(even though the Celts never reached Britain, the Firaxis myth said they did)
Nope. I definitly did not want to give an obvious first clue.
Clue 2: Near this city two (originally) nomadic empires confronted each other with as result a civil war initiated in the empire which is in Civilization V
Ankara? It's a capital today and I think their was a celtic tribe settled in central anatolia in the ancient area. And there were several battles around there between mongols and seljuks/osmans later.