(sorry for not just editing my previous post, but I can't find how to do that :P)

I just tested the upgrade requirements with the walls building tree.

I tested:

requires factions { rome_republic, }
Worked as expected
- Only Rome can upgrade walls

requires hidden_resource not_used
Worked as expected
- Walls can't be upgraded (no region has this HR)

requires hidden_resource rome
Worked as expected
- Walls can only be upgraded in the city Roma

requires not hidden_resource italy
Worked as expected
- Walls can only be upgraded outside of Italy

requires building_present_min_level core_building governors_palace
Worked as expected
- Only cities with a core building "Governors Palace" and higher can upgrade the walls

:)