Commanders are single units with no fighting ability, but they do confer abilities on your infantry. Using commanders makes it much easier to maneuver your infantry brigades and keep them organized. Commanders in close proximity also confer morale bonuses upon infantry troops, making your line much more stable. They also enable the use of certain special infantry commands like charging the enemy and holding the line at all costs. Finally, since they are horse mounted, they can be used as scouts to determine your opponent's force and location.
Artillery also plays an important part of the battle plan for any commander.
Certain types of artillery must be fired from close range while others are more effective from longer range. Artillery fired into the flanks of the enemy is particularly devastating and can break even elite troops very quickly. However, artillery is very ineffective under direct infantry attack and in extreme cases, your artillery can be captured and used against you. Nothing changes the outcome of a battle faster than captured artillery. Each unit is designated with a flag and this flag tells the tale of the status of the unit.