well, actually. i think it woudl be nice to change that ranking.
village-minor city- city- city ('era' 2) city(era 3)
at lvl 3 you get all teh untis the factions ahd for years. and after 'upgrades' you get the units the 'factions' used at that era ( each era could be 'calculated' by measuring how long it would take to build that city)

this way romans can train phalanx triarii from 'the start' but after 1/2 era's the loose formation triarrii will apear(, with better stats-> technological advances)