That's a very "vast" question, geographically and temporally speaking...
Romani and Keltoi fought eachother, side by side, even side by side against more Keltoi or employed by other nations to fight the Romani, for a period of 400 years and even more, from Iberia to Anatolia and from Britannia to Africa...
They influenced eachother immensely and as for you question they slaughtered and got slaughtered...
If you would like to know about a certain region and period, we can narrow the answer, but as Keltoi against Romani is almost all the battles the SPQR had in its history :D
As for the game, exploit their charges and scare abilities :)
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