That means keeping plenty of food – and a lot of food variety as well – and water. But if you do, don’t forget to keep high morale, otherwise, you will be faced with a lot of deserters. Note: If you are vassal to a king or are a king yourself, then you should know that it is possible to attach ally armies to yours.
You should really invest some time into exploring each kingdom’s soldiers, and recruit the ones who have potential. Good armies need good soldiers, not peasants and farmers. Before moving on to worthy fights, battles and sieges, you really need to think about increasing your party size.