The reason why campaigns are so much fun in the beginning, and boring in the end are the battles IMO. The battles are so much more high stake in the beginning, and so much more fun as you can't just auto resolve. As soon as you're in the stage of just "Auto resolve, win, auto resolve win", you really get bored since fighing is a bore, but just managing is a bore, and there's no real big enemies. Medieval II kept me having fun until the end due to the Mongols, Timurids, and Aztecs. They really are end game bosses.
And there's so many enemies and factions.









