I have tried twice to get to the end, but in both games, the last attempt with 7 kingdoms at war with me, I only had enough money to buy off two at half a million each. My relations with enemy Lords and Kings was up to 90% in some cases, but it did not matter.
As you expand, and you are attacked by just three Kingdoms is bad enough, and I found I was retaking castle and towns all the time, but with 7!
Without diplomacy, or some sort of limit on how many factions can attack, it's hopeless. It seemed as my size and total army size grew, (bigger than most, only one faction, Northern Empire had more), multiple Kingdoms attacked. I had a couple of vassals seemed to just disappear, as I saw no notification of capture.
It was fun, but you have to accept the limitation's, there is a lot of work yet to be done, but I do accept some will find the lack of triggers towards the end to limit aggressive Kingdoms with no way to barter frustrating due to massive sums involved. One thing I do miss is the ability to hang on to Lords captured without the system bartering them back, sometimes multiple times in a game day without my interaction.