Well first, they're not cities, they are called colonies.
Second, the easy way to describe this is as follows,
If you are conquered, you cannot conquer another colony, only your conquerers colony.
If you are not conquered, you can only conquer non-conquered colonies.
The only way to conquer their colonies is if their conquer first becomes conquered, then they are freed.
When a person becomes conquered they do not loose any outposts unless they are also attacked.
A conquerer can conquer more than on colony.

MIA