You may give stone wall to the Romans but also to the Celts. We have in project for Gallia : Total War to make oppida wall (mixed stone and wood). You may do the same.

Check our units for Gallia : TW also, it will give you some ideas (thought insular Celts may looks like more 'savages', for the Northern Tribes, with tatooes and this kind of stuff).

https://forums.totalwar.org/vb/showthread.php?t=48638