In my Romani world conquering campaign I had several, next to a city but also on the open plain. So far only one did disappear, a decisive but easy victory near a lake, where I defeated the Aedui.
It's always nice to see whole tracks of heroic victory's, again and again seeing my greatest heroes live on to eternity!
Anyway, the easiest way is as Centurio describes it, besiege a city and then fight a defensive battle where you destroy the relieving army and the city garrison together.
Good luck!
Bookmarks