There were celtic mercenaries on every Hellenistic power that was. They influenced heavily the Hellenistic states of the era. The old round "hoplon" full bronze shield was gradually phased out, in favor of the celtic thureos. They were influenced in return. Those living near Hellenic cities/states or those who had fought with Hellenic armies would arm themselves thus, and fight in that manner. Greeks had a great deal of respect for the Celts... Probably after the fate of Ptolemaios Keraunos, who fought them...