Make backup copies of the files in case it doesnt work
Then, first thing is to remove all the walls which are built when the game starts.
This is the descr_strat.txt file and you are looking for any lines which contain references to walls except for those regions you want to keep walled eg:-
Code:
building
{
type defenses stone_wall
}
Next you need to edit the export_descr_building.txt file
In our files, the stone wall is available at city level
Code:
stone_wall requires factions { all, }
{
capability
{
wall_level 2
tower_level 1
gate_defences 1
gate_strength 1
}
construction 10
cost 96000
settlement_min city
Change settlement_min city to settlement_min large_city
and now the stone wall cant be built until you have the large city
Save files after changes
Not sure if this is save game compatable and remember to delete you map files otherwise the changes wont show up
Hope this helps