My suggestion after playing and loving this little game.
Regarding the military it seems that you can only help your ally and always win. What if you lose? Also, what if your kingdom is attacked directly and you lose?
For example: your kingdom is attacked and you have the choice to give gold to improve your forces (increase chance of success by, let's say 20%, from an initial chance of 35%). If you lose, you would have to pay a weekly sum of gold to the winner kingdom. After you lose, perhaps the knight can offer a chance for liberation. If you win, you can perhaps gain gold and lose people.
Another idea would be to attack other kingdoms with the same mechanic, inverted.
Not to mention you can add a chance for the devil to appear and offer another bargain for souls (example: give 30+ souls to win a battle etc.), or for a chance for the knight to appear after the war started and offered to lead to troops (gain extra happiness and increase chance of winning).