Once formed up, armies can rally into movement formation and then move. They can move two nodes a day (as though on foot).
If they come to a town or city, they remain 'outside the walls' unless either allowed inside by a mayor or they fight their way in. Once inside a town, they are able to loot the treasury, topple the mayor and put in place their own if they wish. If they find themselves within the walls of a capital city an army can take an additional day to try and take the castle. If they succeed, they can loot the county treasury and replace the count and council. An army inside the walls can also choose to defend the walls against further attack.
Combat occurs if an army's settings (controlled by its leader) results in an attack. If it fights another army, a stationary army gains a defensive bonus against one that is attacking. If both armies are moving it gets technical. Town and castle walls give further bonuses. These bonuses affect the basic odds, a function of individual's fighting co-efficient (itself a combination of their strength, weaponry and whether they have a shield).
It can get complex if there is more than two armies involved at the same time.
54 minutes ago