If memory serves me right, you'll need to edit 2 files (actually only 1, but the 2nd one makes it right).

Open bi/data/export_descr_buildings.txt

Look for the 3 seperate lines:
stone_wall requires factions { barbarian, carthaginian, eastern, hun, roman, }
large_stone_wall requires factions { eastern, roman, }
epic_stone_wall requires factions { eastern, roman, }

Basically, if you wanna make large walls and epic walls available to the other factions, just add them in. Eg: making large + epic walls available to barbarians:

large_stone_wall requires factions { eastern, roman, barbarian, }
epic_stone_wall requires factions { eastern, roman, barbarian, }

Don't forget the comma (,) after the faction name.



The 2nd file you'll want to edit would be bi/data/text/export_buildings.txt

Look for the sections under the walls you want to give to the faction. Each buliding type has 2 forms of descriptions:
stone_wall_descr
stone_wall_descr_short

Look under large_stone_wall_barbarian_descr. It says:
DO NOT TRANSLATE! WARNING! This text should never appear on screen! LARGE STONE WALL

This is what you'll want to edit. Simply cut/paste the description from the factions who can build the wall. This way, when you build the wall and right-click to inspect/read about it, you have something there.

Think the above should do it right for you.

Good luck!