Subordinate cities become available at Keep 11. Their major benefits are buffs, debuffs and general EXP -- not their own gold/troops. Buffs apply just by owning a sub-city, scale with rarity, and stack across cities of the same culture; occupying sub-cities on the map grants an additional buff per city. Buffs depend on culture. Battle cultures: Japan = main-city attacking-troop attack (marching buff) + Research Factory material speed; Russia = in-city troop attack + trap attack; Arabia = hospital capacity + healing speed. Development cultures: Korea = extra resources from gathering + warehouse capacity; Europe = construction speed + general EXP from monster kills; China = main-city resource production + training speed; America = current sub-city gold production + research speed. Concrete numbers: hold up to 9 sub-cities; 9 Japanese Epics = +90% attack; 9 Korean Epics = +135% resource-gathering (finishing a 7M tile yields 16.45M). The in-city sub-city buff differs from the Culture Feature buff. Change the in-city sub-city culture by changing your main city's culture. Debuffs: sub-city troops are weak -- instead assign a mayor general with debuff skills (attack/HP down), which stack across mayors (some reach -600% with equipment/specialty); Ares/Achaemenidae debuff equipment and the Norway Ridge dragon amplify this. Mayors earn half the main general's EXP. For an economy bot, Korea (gathering+warehouse), China (production+training) and America (sub-city gold+research) are the development-culture picks; combined with Champion's gear and gathering research they multiply the free gathering lever.
Original research source: Evony Guide Wiki