Well....
First thing to do is decide what each army has. You need to take in account the type of land the battle is going to be on. Open plains, rolling hills, and other wide open spaces plays into the hands of Calvery and archers. On the other hand, mountain passes and other confined spaces are more for massed infantry.
Now, there are different types of Infantry and Calvery. Basiclly, you have Light, medium and Heavy units. Light units are scouts, harassing the enemy and slowing him up. They cannot stand up to an all out attack by heavier units. They can move fast and can bed all overt the battlefield. Your scouts are light infantry.
Medium units are a combination of speed and firepower. Slower the light units, they have hevier armor and weapons then light units. Heavy units are usually the elite, the career soldiers or nobility. They are slower then the other units, but they can ground up the others.
There is a series of PC games called Total War. Right now, there is TW: Shogun, TW Midive, and TW: Rome. The key part of the game is the battles, as you army goes up against other armies. You have to chose what type of units to put in your army (Depending on your location and tech level).
Go to this web site: http://www.totalwar.org/strategy/twug/index.html And it has a description of all the different military units in the different games. It'll give you an idea of the different units you can incorprate into your story.
Craig
I'd actaully argue with a few of those.
You can have heavy infantry, which are more like your roman legions.
And light cavalry make excellent scouts.
I would argue very strongly against using a game like total war for your setting and units, because they gloss over a lot of important minutae.
Yes, but the game does give you an idea of the type of units you can have. I mean, disciplined Pikemen will stand up to a calvary charge while peasents will break and run if pressed by the same calvary. Archers can cut armored knights apart under the right circumstances (Angencourt comes to mind), but if the knights get in amoung the archers, well, the bowmen are toast.
The type of country affect the type of army it has. A nomad nation will not have pikemen, for example, because they are not liable to have the inferstructure or the need for them. Deciding what sort of an army a nation has is part of the world building every author needs to work out. History, the landscape, the type of goverenment, all have an influence on the military.
I suggested the game to get a quick and dirty overview of what sort of units an army could have. If you really weant to go deep, Osprey Publications is based on military history and military of all the ages. But unless you're deep into that sort of stuff, it's too much info......
Craig
I'd have to agree with LightSabre when it comes to using a video game as a reference for a story. I'm a big gamer in my free time, and games can be very unrealistic for the sake of ratings and coolness. I've never tried my hand at Total War, but I've played more than a few RTS games in my time and a lot of it is about upgrades and health points.
Plain archers don't have a chance against mounted, armored men. Arrows don't have to penetration speed to punch through armor. (yes, lucky shots in exposed areas are true, but not all archers are lucky all the time.) That's what crossbows are for, though they are slower to reload and have greatly reduced range. As Trboturtle said, though, once the cavalry get around the archers, they're toast.
Also, after taking a quick look at the Total War guide, it appears to have one thing most RTS games have, which is upgraded units, such as 'Mounted Sergeants'. Another thing that wouldn't happen in real life is people who can suddenly take a hit better than the guy next to him because of rank. Does that mean that if we send a 5 Star General into the battlefield that he can take an RPG in the chest, but his rank will save him? He may be covered in war medals and brass, but that won't help him.
Leaders are, of course, necessary, but remember they are normal people, too.
Will continue this later. Gotta go.