Uh I wouldn't
I'm pretty sure they (TW) don't want Kingdom destruction. Everything about this game is made with the mindset of there always being the same 6 Factions/Cultures present so there's a constant state of war (even though none of it makes any sense).
I don't have a problem with Kingdoms being hard to destroy, many times Kingdoms/Nations have risen from the ashes in reality. IMO it would be better if when a Kingdom loses its last fief it effectively becomes a Minor Clan at permanent war with whoever "destroyed" them. Problem is Minor Clans can't take fiefs, so they'd have to do some work to make them unique. That or they become Claimants - but hey that's totally missing from this game as well!
I get that a fiefless faction basically ends up as "terrorists" extorting actual Kingdoms, but doesn't really jive with how tribute/reparations is calculated for rest of the game. The other problem is I'm pretty sure all these Kingdoms have a secret piggy bank. If a rogue Kingdom is giving you trouble; you should be able to effectively capture and lock up the "rebels" indefinitely. Or you know kill them. Don't know why this game has an execution mechanic since it's totally unused. And using it just once as the player basically makes you Public Enemy #1 of Calradia, at which point you basically have to embrace the Dark Side.
Late game just isn't terribly well thought out. Once you reach a certain size like 12,000-15,000 Troops you can pretty much steam roll everyone into submission as long as you the player are calling the shots Kingdom wise to stop your vassals from being idiots. Strat proves this:
So I guess there isn't a
need since you basically reach a certain point that may as well be "Victory"
Game would be a lot better if say Kingdom ruler could give Armies objectives. Also the game desperately needs some form of basic Diplomacy
- Having some kind of casus belli and again objective for a war (i.e. take a particular fief, weaken a Kingdom, stop paying tribute, etc.)
- Having actual Truces/Ceasefires
- Kingdoms should form Alliances against any single Kingdom that starts becoming too powerful
- Actual explanations for paying Tribute, etc.
Honestly because there's no cultural conversion in this game, I would just hard program every Kingdom to more or less focus on maintaining it's defacto fiefs with war interest fading with every non-culture fief taken. Of course cultural conversion could make for a somewhat interesting late game mechanic (imagine if you could theoretically make eveyone Khuzait heh) - though it should obviously be a very slow process. Like it should take 10 in-game years to convert a Castle/Town and you can't lose it during that time, otherwise progress is reset. And make it progressively harder. Like converting the first Battanian fief might take 10 years, but the last one should take like 100 years.
Just strikes me as weird the "Calradic Empire" was able to convert existing peoples to become "Imperial", but this sort of feature is wholly missing from the game.