The problem with the term 'civilization' is the fact that historically there is near universal tendency of settled, urban societies to discriminate against peoples they perceived 'uncivilized and therefore greatly inferior'. Though maybe I should rather write 'near universal tendency of all human societies', because even very 'primitive' (another very unpleasant word

) societies had their own category of 'barbarians' and 'uncivilized savages'.

Therefore, it is impossible today to describe any historical group of people as 'uncivilized' without sounding like a jerk. I have encountered Australian articles on some Aboriginal sophisticated hunting techniques as a proof of 'Aborigines being the oldest continuous human civilization'. With all respect, if we can describe Australian hunter - gatherers as a 'civilization' then we may as well throw this word out of the window, as any and all human societies become Civilizations from similarly humanist perspective.
Personally I am quite attached to the classical definitions of 'civilization' as a
settled, urban, agricultural society with a non - tribal government and social hierarchy. Writing is quite silly requirement of a civilization, because there were some very big and advanced societies without confirmed writing systems (most notably in Andes, but also some Subsaharan civilizations). Similarly 'organized religion' is a weird requirement which smells of some eurocentric visions of what is deemed to be the 'proper' religion (especially as even beliefs of very small and native human groups tend to be very poetic and sophisticated). Currency is obviously not required either, as it was only discovered in 6th century BC.
This would immediately disqualify Maori, Mapuche, Cree and generally all Native American tribes. Personally I'd be perfectly fine with this, as I have always felt Native American tribes just feel awkward on this scale. Generally I don't think a group of people should be in this game if its description in encyclopedias is more anthropology rather than history.
Then we get some problematic cases which are hard to classify using the classical binary notion of "civilizations vs tribes". Mongols are an interesting example, because they were basically nomadic tribes which due to very specific factors were capable of becoming an enormous power on the "macro civilizations" scale, so they get a special pass. What helps here is the fact they actually were capable of governing cities and states once they conquered them. Scythians, Huns and Manchu probably also get a special pass for similar reasons - if a group of nomads was so big, so long living and so powerful that it terrified empires, then it is an acceptable exception from the rule.
Then we get cases like Celts/Gauls, where they seem to be in the very interesting "proto civilization" stage, halfway between "tribes" and "civilizations". They lived in a countless mass of tribes, but those tribes had population centres comparable with large cities; they had roads, mines, diplomacy, organised armies, advanced metallurgic industries, irrigation, long range trade and some of them even minted their own coins. Such peoples seem legit for me to arrive into games like this one.
But in the end it doesn't matter, because today every group of human beings is a 'civilization' in discussions like this one, because we don't wanna offend anybody due to the unfortunate implications of calling anybody 'uncivilized'.