probably can't be done but seeing as Undead usualy get their resourses not from a resource itself but from leftovers of other armys I think Undead resources should go like this:
if in cultural borders only a city working it can get that resource and only as the that city normal bonus for working the resource.
can't trade resources.
gets 1 resource for every 1 of that resource 2 other civs has.
that way to get copper weapons 2 other civs have to have copper and for iron weapons 2 other civs has to have iron + of course the undead civ needs the forge to upgrade in that city.
Undead are usually summoned with used weapons of other armys and don't tend to get upgraded so maybe:
Undead summoned before 2 civs have copper, iron or metiron have no metal wepons and can't upgrade to any.
Undead summoned after 2 civs have copper but before 2 civs have iron or metiron have copper weapons no chance to upgrade.
Undead Summoned after 2 civs have iron but not metiron have iron weapons no chance to upgrade.
Undead summoned after 2 civs have metiron have metiron weapons.
and resources are normal so other units upgrade as fast as the Undead civ can get the metal and forge.
this gets a new what do I want to do thought process going. I just got multiple metal resources so do I want to trade some so 2 other civs have it and my new Undead have better weapons or do I want the other civs to not have the metal more?
if in cultural borders only a city working it can get that resource and only as the that city normal bonus for working the resource.
can't trade resources.
gets 1 resource for every 1 of that resource 2 other civs has.
that way to get copper weapons 2 other civs have to have copper and for iron weapons 2 other civs has to have iron + of course the undead civ needs the forge to upgrade in that city.
Undead are usually summoned with used weapons of other armys and don't tend to get upgraded so maybe:
Undead summoned before 2 civs have copper, iron or metiron have no metal wepons and can't upgrade to any.
Undead summoned after 2 civs have copper but before 2 civs have iron or metiron have copper weapons no chance to upgrade.
Undead Summoned after 2 civs have iron but not metiron have iron weapons no chance to upgrade.
Undead summoned after 2 civs have metiron have metiron weapons.
and resources are normal so other units upgrade as fast as the Undead civ can get the metal and forge.
this gets a new what do I want to do thought process going. I just got multiple metal resources so do I want to trade some so 2 other civs have it and my new Undead have better weapons or do I want the other civs to not have the metal more?