For those who are crying that one developed fighter can be brought down by two noobs with some combat skill, I remember that this was exactly one of the main points to level out the old combat back in world 7 when we had 1000+ UA juggernauts that could raise a whole village to the ground and the only thing that could stop them was another such juggernaut. Besides as was said most people never master pvp combat tactics because you can only do that in actual combat, so besides large fighting oriented factions that regularly commit time to training among them selfs no one else has the means.
One other point, in the medieval times fights also usually lasted as long as they do in game. This is a crafting/survival mmo and not wow/lol/counterstrike/whatever fighting should be the last course of action and not the first thing a developed character does when encountering a living soul in the wilds.
EDIT
Two RL hours after a Border Cairn has been challenged, it can be attacked and destroyed. Destroying a Cairn takes a fair amount of time ~30 minutes depending a bit on your strength and whatnot, and only one person at a time can attack the cairn. Border Cairns cannot be destroyed without a challenge. If you destroy a Border Cairn (which is not yours) your realm gains 5000 Authority.
So two kingdoms can come to an agreement to destroy and rebuild each other border cairns without any fighting and thus gain, lets say 50k authority each, daily.