Battles may occur either as the result of a raid or an invasion. There are three types of battles: dispersal, open field, and set piece.
Dispersal: Dispersal battles occur only as the result of a raid. This type of battle occurs when defending troops pursue and engage single troop units of a raiding party which has dispersed and is attempting to flee from the province it has raided.
Open Field: Open Field battles may occur in both raids and invasions. This type of battle represents unorganized fighting across a broad front. Such battles are a confused swirl of engagements stretching out over miles or perhaps even hundreds of miles of territory. Open field battles will involve entire opposing armies without permitting either side to draw up into classic battle formations. All sea battles are always considered open field.
Set Piece: Set piece battles are classic engagements between opposing armies arranged in battle formations. Such battles occur as the result of a raid or an invasion. In raid situations battle formations are arranged helter-skelter hours or perhaps moments before the onset of battle. As such, you will not personally decide the formation; the character who leads the raid will choose battle formations based upon the qualities and abilities he possesses (if you don't have a character in the raiding party, good luck).
In most set piece battles occuring as the result of invasion, the battle
formations of the opposing armies will be laid out with care and, perhaps,
days of preparation. This will involve an extensive period of maneuvering,
and your last chance to assign an army commander. Following maneuvers, you
will arrange the disposition of your troop units into battle formation. Such
battles are prolonged and bloody affairs upon which the fate of empires may