An army must have a general attached to it.
Generals may be free samurai recruited into your clan, or they may be your adult male family members. The minimum age that is required to be general, is 15 years old.
Generals may die of disease, old age, or be killed in battle.
Generals may learn certain traits which enables them to be better fighters.
The standard formation of an army is the ganko (birds in flight) formation. When you attack or defend with your army, you can pick one of three more formations.
The 3 defensive formations are: engetsu (halfmoon), saku (keyhole) and koyaku (yoke). The engetsu formation is best used in a desperate situation when you are hugely outnumbered. The saku formation is the best formation against a fierce charge. The koyaku formation is a more flexible defensive formation and is somewhat inbetween the engetsu and saku formation, allowing the defender a chance to take up the engetsu formation or the saku formation.
The 3 offensive formations are: gyorin (fish scales), hoshi (arrowhead) and kakuyoku (crane's wing). The gyorin formation is basically a blunted charge and is best used, if you wish to keep the attacker's casualties low, in comparison to the hoshi formation. The hoshi formation is a fierce charge and designed to crack through the defender's lines, it causes increased casualties to the defender when used with a succesful charge. The kakuyoku is the best formation, if you wish to envelop an enemy army.
When you create a new army, you must have a general attached to this newly created army. Free samurai may be used to create an army at whatever castle they can be found. However members of your family can only be deployed at your capital. Hence you must pick one of your sites to be the capital of your realm, if you wish to use a family member as a general.
Your army units are put into a reserve when the general of that army dies for whatever reason, either from old age or due to assassination. This reserve can be added to one of your existing armies.