For detailed unit's statistics go here: Levy (with stats)
For influence of terrain on the battle see Combat and Terrain
For detailed description of order of battle, flanks and combat see Combat & BattleWhen you are preparing for a war or a serious battle, it helps to stack all of your levy from different provinces into one or few powerful armies. The bigger the army, the better chance of winning a crucial battle.
There are three positions in your army - left and right flanks, and center. Each flank is commanded by a character, or may be not.
When clicking on your army, you can reorganize your levies and the flanks at which they are positioned by clicking a little arrow on the right side of army display window (see screenshot). The window will expend displaying in detail what groups of troops are located on which side.
Therefore, you can position your heavy infantry and troops, let's say, in the middle, and your cavalry and light troops on the sides. You can move your soldiers to the top or buttom inside of the flank or center itself, reportedly to bring closer or further from the front of battle.
Players have reported that re-organizing your army can bring very tangible and real success in battles.