Pick off your former allies, and sue for peace.
Wait till you can break off on your own, then when they try to siege, place the men into your party as they will spawn first in the siege, and you will win if you play it smart. You can now take a castle, and wait for it to be denied, if it isn't then just take another/ let it be captured and repeat. Train 250 siege troops (Ravernstern Rangers and D'Shar Bladesmen are my go to) You should focus on getting 600 mid tier troops in your castle (If you want to exploit the AI, then just place 3000+ recruits.
Hopefully this worked, If not, just wait for them to recapture it and try again. Hopefully you have good relations with your marshal and you can tell him to defend the castle, or you can kite a army into the enemy to eliminate them/weaken them. When you take a castle, recruit as much men as you can find to hold it (You should be able to get about 200 before enemies arrive). Make sure you can carry a party of 200 men before trying to create a kingdom. Ensure you have 250k, as your early kingdom you tend to lost 20k per week. If you can't afford it, enter tournaments. Ensure that you have the most profitable enterprise in every city.
