Generally, tag synonyms aren't used for significant substring matches. This would make city-design a poor synonym for city, as if the two are separate anyone typing city into the tags field will get suggestions for city and city-design. An example of a better synonym could be, say, village or homestead (none of which exist at this point) to city, since (say) someone who types village wouldn't otherwise know that the proper tag is city.
I haven't looked at the two tags in detail, but I do note that we have at this time:
I don't see any obvious reason why we should have both, however, so I'd be inclined to just merge the two tags. Since there are so few questions involved, this is something the community could easily do.