List of IDs that have an assignement to ALL other Ids in another table
You can try something like:

select a.assignmentID, c.city
from #cities c
join #assignements a on a.cityId = c.cityId
where c.cityId in ( select c.cityId
                    from #cities c
                    left join #assignements a on a.cityId = c.cityId
                    group by c.cityId
                    having count(*) = (select count(distinct assignmentID) 
                                       from #assignements)
                  )