Infantry is the backbone of most armies, a bone that if broken leaves us vulnerable. A simple way to ponderate whether an infantry unit will lose in melee against another is to compare their costs: expensive units beat cheap ones. When the price of units doesn't differ much, another factors come into play; exploiting them is often key to winning battles against other players.
Beyond the obvious, there are some more details to keep in mind to use infantry well:
- Units have different masses depending on their armor and training, but this isn't very important when fighting other infantry. Instead, the mass becomes more relevant against horsemen, elephants, chariots, (monsters)... The more massive a unit is, the better it will stand the charge, and less damage will suffer.
- The charge bonus is a determining point for the outcome of an infantry combat. It is why barbarians can crush Roman legions. Keep it in mind, take advantage of the charge bonus of your wildest units with clean charges and protect yourself from the charges of the strongest units of your foe.
- There are two types of damage, basic damage and penetration damage. Penetration damage ignores enemy's armor. It's that simple. Choose your weapons depending on your enemy. If you face heavily armored enemies, grab the axe. If you face naked euphoric Celts, a sword will suit you better.
- Numbers matter. A unit that has enemies in front and to the sides will suffer more casualties because it will receive hits from many directions and there will be men (or women) who will have to fight against several enemies at once.
The base of most armies. Use them against enemy infantry, but beware of their cavalry. Swordsmen with javelins, like Roman legionnaires with their pilum, can do a lot of damage to horses when throwing them.
In general, a unit of swordsmen with a cost x will defeat a unit with lances or halberds of the same cost.
Some infantry units with swords have shield formation; it is convenient to activate it if you foresee an enemy cavalry charge. If, otherwise, it is an infantry charge that is coming, it is best to charge ourselves as well.
Axe bands are similar to swordsmen units, but with greater penetration damage and a better charge bonus, although they tend to have less armor. Very useful against the enemy's most armored units, but remember to always be the one who charges.
Spearmen: the base of many armies. They are the most flexible infantry units, thanks to their ability to kill cavalry and their big shields (almost all spearmen have shield) that protect them from arrows. Try to have spearmen units close to your swordsmen or axemen to protect them from enemy cavalry. In addition, they defend the flanks very well. Their weak point: they will almost always lose against the enemy's swords (unless your spearmen are significantly superior to rival swords).
Use them as a first line against armies that stand out in cavalry and missile. Against armies with good melee infantry, use them as support units.
A kind of midpoint between swords and spears. They are better in melee than spearmen, but they have no hands left to carry a shield, so arrows are their worst enemy. That's why they work best in armies that are powerful in ranged combat, which can finish off enemy archers before they can shoot the halberdiers.
Alexander's anvil. I wonder if antiquity pikemen where as powerful as they are in games. In any case, they have a big weakness: their terrible maneuverability; a player with some experience will play with them like spanish 'toreros' play with bulls.
Very useful against AI, not very useful against other players. They are still playable, but you have to know to make the enemy go to them. A strategy in which I use them consists in investing many resources in one flank to ensure victory in that flank and then sorround the enemy to attack from the rear. The way to prevent the enemy from sending more troops to defend the flank with few resources (we have invested a lot in the flank) is to cover the center with cheap pikes that advance slowly.
I don't mean that this is the best way to use pikemen, it's just an example in which I take advantage of their strengths.
Dacian falxes, long axes and other two-handed weapons
Great charge bonus (unless you're a dwarf) and damage but weak defense. It's not worth it to have a lot of them in the army, they're more adequate to flank and attack enemy units already in combat. On the other hand, well used provide a huge tactic advantage, because they get rid of enemies fast. Two or three units of this type charging against units inferior to them can open in a short time a breach in the enemy lines. If you want to test it, incorporate a pair of these units in a line of armoured swordsmen that can stand a long melee. My favorite strategy with Carthage is similar to This.
Blocking savage's charges
If we play with armies like the Roman and face barbarians (e.g. Boii, Lusitanians) we have the problem that our units have little charge bonus and theirs a lot. This causes that our units will suffer too many casualties in the first shock, even if we countercharge. The solution is to hinder the enemy's charge.
A way to block these charges is to put some cheap skirmishers in front of your main line. When the enemy is very near, charge with your infantry, but do it in a way that the first shock is received by some fleeing skirmishers, but not too many of them: they are cheap but you don't wanna lose 'em all.
Other option is to use cavalry to charge enemies first and break their lines, and then incorporate infantry while retreating cavalry. This is a difficult maneuver, but in some situations very helpful. Anyway, anything you come up with that can hinder the enemy's charge can work; it's said that necessity sharpens ingenuity.
Finally, in Three Kingdoms, elite sentinels bring the ability to block charges to melee infantry under their command. Although I don't quite understand how it works against cavalry, I've seen that it reduces the damage they receive from the charge but they still receive a lot of damage.
Relieving fatigued soldiers
A very important point, especially in siege battles or settlements. Exhausted units fight worse than those who aren't, so if a unit is very tired and we have fresh units in the rear, it's a good idea to exchange them. There's an important point to avoid casualties in the procedure: first join the fresh unit into combat and then retreat the tired one. In this order.