In RTW was where I first valued the missile units, at watching a horde of cretan archers decimate a roman army, a battle in which I was outnumbered 3 to 2, but still managed to only lose 10 men tio their 500 or so. I noted their destructive froce in seige defense, and fell in love, so to speak.

I found that if you place in front of your infantry at the onset of battle, you can wreak havoc on the enemy soldiers as they mach to greet your men. Once they get too close, you place them behind your infantry and turn skirmish off. Then, you fire at the enemies rear ranks while their front ranks fight your men (to reduce friendly fire). When you run out of ammo, send them away from your ranks. They are easily protected by cavalry, and both cavalry and your archers themslves can generally take on an infantry unit that snuck around your line (very rare, especially if you use proper flank support.