yep, open up the export_descr_buildings file and first add walls as a hidden resource:

hidden_resources sparta rome italy walls

Then beside each wall entry further down the text file add the following:

and hidden_resource walls

For example, for the wooden pallisade:

wooden_pallisade requires factions { barbarian, ct_carthage, eastern, parthia, egyptian, greek, roman, } and hidden_resource walls

Then open up the descr_strat file and remove each wall entry from each city!

And away you go! :D