Before you can begin mass combat, we must have our armies first. Building an army is simple and can be done quickly, especially if an army is built off a single type of creature, like an orc warband or a kobold army led by a ferocious dragon tyrant. Or, maybe this army is a bit of a hodge-podge and you don't have a single creature to build it off of, in which case we will go over how to build an army from scratch.
Skills are the measure of an army's abilities and how they function on the battlefield. These skills help an army seize a wall, pass obstacles, trick opponents or heal the injured.
Every army must be fielded with equipment, even just the basics, though the basics won't make them very effective. Every army starts out with the following proficiencies.
An army is made up of several different units, all working together to siege a castle, defend against an orc warband, or to conquer the world. With this system, it is not ideal to field a lot of specialty units, instead there should only be one or two such units at a time split off from the main army and only if they are doing something tactically special from what the main army is doing.
Spellcasters are a powerful tool in any army, though they are only effective when they are used in large quantities. While a single spell can decimate a large area, armies are large enough to recover from a single spell launched at them.
An army can differentiate itself from another by special abilities or the ancestral make up of its army. Some armies, like kobolds, are sensitive to sunlight, where as orcs are naturally intimidating to face.
When a kingdom wishes to assert their dominance or lays a claim to a castle that is currently not theirs, they must go to war. Armies are made up of multiple units, but this is abstracted out into a single entity unless specialized units are formed.
An army that is constantly fighting and not paying attention to the well being of their troops finds it harder to field their armies as well as keep the men from running during battle.
Defending a keep, a city or a fortress is a major use of most armies, with very few attacking others.
Weather can play an important part in determining who wins a battle. If heavy rains causes the ground to turn from hard pack earth to 3-foot deep mud, the army won't be able to bring up their siege equipments and their calvary won't be able to move quickly across the field of battle without serious risk to their horses.
Armies are only useful if they are able to travel, and travel quickly. The more time you provide a city to bring up their fortifications, the harder it is to bring down the walls and crush them under your boot heel.
Taking care of an army is a difficult task that requires a lot of funding and supplies. Soldiers work for money, though fanatics might be persuaded to join a rebellion with no pay, and soldiers require food or they'll starve, though undead and constructs don't have to worry about that.
Siege engines are used for the purposes of assailing a fortification and require a specialized unit of Siege Engineers to construct them. An army without siege engineers do not add their proficiency bonus to attack rolls, ability checks or saving throws dealing with siege equipment.
The following are sample armies and specialty units. These can be used as made or simply used as inspiration for how army stat blocks can be made.