The Barbarian culture in RTW didn't originally have as high settlement levels as it does in BI - if you wanted them to have the higher cities in RTW based mod you had to do something like
How to Transfer Barbarian Buildings from BI to RTW was that what you were trying to achieve? If so see if that tutorial gives any clues.

If you want to use some of the BI buildings I think you will need to copy and paste the entire BI/data/items folder into your mod folder and also use BI version of the descr_items.db file. That has RTW stuff plus the extra BI bits in it so you should then have everything... (the items folder has a funny quirk that you can't split contents between mod-folder and RTW level - it all needs to be in mod-folder if you want anything read from there)