I actually got the idea watching a game and remembering abt your mod having it.

Not sure how many times I have asked, suggested you play Classical again, but yes we have them as teritory roots.....

I mean,historically forts were where many last stands and guerilla warfare happened. I think they should be root.

Ok, I’ll start with either city states or tribes. would it be safe to say in general all tribal buildings are mostly wood? some other ideas on what they may all have in common? 

The categorisation cannot be clean cut, because it mixes forms of government with sizes, which is related to how heterogeneous they were. Taking what they were for the most time (Macedonia was an Empire for a very few years): Tribes: Britons, Gauls, Iberians, Germans. City-states: Athenians, Spartans, Thebans. Republics: Romans, Carthaginians. Kingdoms: Macedonians, Ptolemies, Kushites. Empires: Seleucids, Persians, Mauryans, Han.