Positioning units for bridge defences

[ReluctantSamurai]

Really, though, coming from all sides sounds a lot more effective.

A lot more effective. You get flanking attacks and attacks to the enemy rear when they rout.

Exactly how far do you keep them away?

The best way to figure this out is to set up a custom battle with archers/teppo on the enemy side. After a few battles you'll get a feel for how far to stay back

Another suggestion is to remove the fire-at-will from your archers/teppo. This conserves ammo in large battles and allows you to rake enemy units as they cross the bridge. I usually group my archers so they fire as one unit. As soon as an enemy unit(s) become engaged with my own infantry, I move their fire onto units crowded on the bridge. You cause a lot more casualties that way for the same amount of ammunition.

When facing multiple stacks, I try keep several archers in reserve (if I have them available). Even with micro-management, you'll still run out of arrows at some point. Tuck those archers that are out of arrows to the rear (or withdraw them if you have reserves, though I wouldn't count on getting them onto the battlefield when you need them), and move fresh ones to the front.

In large, multi-stack battles, try and refrain from using ammo on reduced-size units. The AI favorite unit to do this with is CA. It'll prance those dozen or so (for example) CA's all over the place making you use up precious arrows shooting at them. Better to chase them off for good with your own cavalry or just hold your fire and wait for the main enemy reinforcements to arrive. The damage caused by these broken units is usually minimal and not worth tiring out your units by giving chase.

Again, use the custom battle generator to play around with units and techniques.....it's much better than getting your head handed to you during a game because you were unfamiliar with how things work