I can tell you my first tactics. I believe combat is rather standard.
TACTICS
Front: Heavy infantry(any cheap armored infantry) at hold & closest. If you make two groups of them few enemies will come trough since enemies are lured to engage hand to hand if they are fairly close.
Don't use light armored troops if you can afford heavy as they cost the same gold.
mages that buff your troops are welcome here.
Immediatly behind: Missile troops (maybe indy crossbowmen if you don't get longbowmen), javelineers can fit here too and then get archers a little behind.
They will soften the enemy and if they are close will hit more often.
Cavalry: If you fit cavalry or anything faster than your heavy troops (light infantry or militia or flagelants) put them on the flanks to hold and attack rear (or archers) if you are lucky they will pass the main army and hit archers but if it's a luxury type like knights put them on flank and very rear so they meet battle only to chase units that rout so few of them escape.
Example: if you get light units of an event treat them as your first flankers to aid your heavy centre and your cavalry (use heavy and never light cavalry) just to chase.
You can make this armies fairly good if you build castles on good resource provinces like mountains so most of your force is cheap armored soldiers.
using magic is a little trickier but as you conquer you can divert some gold to do it.
Oh, there's another dirty trick. Put a very armored slow troop with feew units at hold and attack rear placed on flank and very foward. They will atract missile fire and survive (with shields of course) if they are wounded better, some wounds slow them down (crippled for example) and they will never see hand to hand because your faster troops cover by the time enemy gets there.
