Elsewhere on these very informative boards is a recommendation to mix archers (specifically English LB) and arbs, this is where I feel that this tactic is most applicable. Once you hash out sizable losses to the AI with missle fire a couple times it tends to get gun shy and backs off your formation if it remains intact. To encourage this behavior I like to mix 2 units of arbs with 4 archers (even LB), withdraw my archers when they are out of ammo and since the arbs have a slower rate of fire, which is still very effective, then the AI registers the losses and pulls back without fully commiting it's forces. If it goes right you'll never need to swap out the arbs, but you'll likely get only 2 sets of archers to run out of ammo. Because this is often time consuming I tend to rush enemy forces and chase them off the map, setting up a spear wall, archers in the rear and cav on the flanks where their reinforcements come in and if my archers are out of ammo, I use them as light infantry. I rout them as they enter the battlefield before they can mass.
When defending with large reinforcement options I always choose the highest ground on my edge of the map. Reinforcements are always quicker there. Defensively, I have got the AI to take such losses that even though all my missle units are out of ammo the AI just sits there and stares at my spear wall, even though if it attacked fully, I would be overwhelmed.
When choosing a flag point, I always choose a forward position, close to my front, on high ground, unless I am assaulting a keep, in which case I choose a point in a treeline (if available) closest to the castle, but far enough that they don't walk in range of fire from the fortifications as they come on the field.
EDIT: I would like to add that if you don't mind LOADS of boring shuffling, you can shuffle the order of units (as to when they become avaialble as reinforcements) by doing 2 things. 1) the order you commit your stacks (move them on the map I mean, onto the contested province/fort) influences where units are in the reinforcement list. 2) take one unit the AI gives you at the start and shuffle it in and out. Shuffle another unit, note as you go through the list where the first shuffled out unit is on the list. It will always appear in that spot. By shuffling units in this way you can set the order of how your reinforcements appear in a fight. I don't do this too much, it isn't fun, but it will make fights easier to plan and win if you know you need more archers or cav to chase routers.
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