What happens when humans conquer an empire city?
Empire cities shall be inhabited by a different race and have different buildings, so what happens if humans conquer an empire city? Shall humans simply always destroy that empire city or shall they occupy that city, allowing them to build empire buildings and have an empire race in one city. Another possibility is that that city is slowly converted to a human city.
You could also ask the reverse question "What happens when the empire conquers an human city?" but I think that it is very likely that the empire would destroy or enslave every human city it conquers.