I also think it would be a good idea to divide things into defensive and offensive tactics. Conducting an open field battle has a completely different set of unwritten rules/tactics when compared to a siege battle.

As well, it would be a good idea to discuss the fundamentals first, such as unit formation and which units types are best used in certain positions.

My battle-line is quite consistant, when possible I have my units in this formation:

---a--a--a--a---
c--s--s--s--s--c
c--r--r--r--r---c

--c/g-c/g-c/g--

a=archers
c=cav
s=swords/spears
r=reinforcement swords/spears
g=generals unit

I allow my archers to fire off a few volleys and then retreat back to the space alloted behind my reinforcements. From there organized chaos takes over and I slaughter my enemy. I have found this formation to work quite well for me in my english campaigns.

One unit which has proven quite valuable for me aside from the famous swordsmen/longbowmen has been billmen and its variants. Whenever a part of my line is crumbling I send in a unit of billmen and their high attack+charge bonus turn the tide of that battle within a battle in my favour. Bear in mind that this is in the early game as I have just started playing M2 and dont have to funds to support an all swords army. For the cost, I cant complain.

I'd really enjoy hearing some critiques on my formation and unit choice, feel free to chew it apart haha