There is a descr_win_conditions.txt file in the campaigns folder. It is blank in EB as they have scripted ones.

I *think* actual short campaigns can't have specific cities as part of their conditions, only outliving x faction.

But as EB has their own scripted victory conditions you can make your own version of a short campaign by adding a record in the blank descr_win_conditions.txt file.

e.g

britons
hold_regions Cassemorg Brigantiae Caledryn Corieltauvae Cornovae Cruddain Cambriae
take_regions 7
short_campaign outlive_factions
gauls

not sure if you can safely remove the short campaign lines but either way you can ignore the game's short campaign and EB's main victory conditions and make your own short version of a campaign.

been meaning to do this for myself as for certain factions I like short campaigns with more limited objectives. so far i've just had those conditions in my head but it's nice to get the actual victory message. haven't tried it with EB yet though so can't say for sure if it is problem-free.