Weather can be an important factor for any given battle. In Medieval: Total War, the attacker determines the day of the attack, and thus the weather for that battle.
One of the most common concerns is rainy weather. In rainy weather the range and accuracy of missile units (such as archers, mounted archers, longbowmen, crossbowmen, etc.) are reduced. In a battle where ranged troops might be a decisive force, rainy weather may be to the defender's detriment and to the attacker's advantage.
Weather conditions can also change throughout the day. For instance, a day may start clear and become progressively more rainy, or inversely, start as a rainy day and clear up as the day goes on.
The High Ground
Holding and fighting from the high ground is extremely important.
For ranged units such as archers, the high ground provides additional range. This means tall peaks or hills can drastically improve the effectiveness of ranged units.
The high ground is also important for melee units. It is difficult for a unit to carry a battle uphill. If a unit is forced to fight from below an opposing unit, it is likely to suffer many more casualties at a much higher rate than it would fighting against the same enemy unit on even ground. This may lead to a fast routing of the unit, especially in less professional militia units. For this reason the high ground is an extremely desirable position to hold.
It is also harder for ranged units to fire up onto units holding the high ground, and as such the high ground also has the benefit of added protection from ranged units for both melee and ranged units.
Ambush and Surprise
While the aggressor chooses the day of an attack, the defender has the opportunity to strategically place troops before the battle actually begins. This gives the defender the opportunity to advantageously place troops in strategic positions, such as on the high ground, but also to disguise the presence of troops by hiding them in nearby forest. Such units are functionally invisible to the enemy until they approach somewhat near those troops.
Done properly, ambushes from multiple sides or deft flanking maneuvers can be engineered in this manner.
Unit Specific Tactics
An important part of tactical planning is the superiority of some kinds of troops against others. For a list of all units with descriptions of their capabilities, advantages and disadvantages, see Medieval Total War Guide/Units.
Cavalry are troops mounted on horseback and are often key to battlefield strategy. Though they are not the backbone of an army as they are traditionally portrayed, they are the most maneuverable unit on the field. This means several things.
The most common use for cavalry is to use them to move around behind or to the side of enemy troops. It is hard for enemy troops to respond to this sort of quick maneuvering as their speed is vastly slower. Even when an opponent has their own cavalry, if they are not deployed in such a way as to let them quickly move in response (for example, if they are on the wrong side of the battlefield or too far behind their own troop lines) then they may not be able to effectively respond at all: The target of the flank may be routed too quickly.
While this speed can be of great advantage on the attack, it can also be very important for defense. In battles of slow moving infantry units, cavalary can quickly come to the rescue of a waning unit with a sudden charge.
The more heavily armed and armored a unit of cavalry is the more solidly it will perform in combat, but its more lightly armed and armored counterparts will move more quickly. Thus, heavy vs. light cavalry is a tradeoff between maneuverability and fortitude.
Some mounted troops can dismount, acting instead as heavy infantry, which provides them a measure of flexibility depending on the situation.
Missile and Ranged Units
Ranged units are units that are equipped with projectile weapons. Examples include archers, crossbowmen, and arquebusiers. These units are usually not very powerful in hand to hand combat. There are many common uses of ranged units. Concerning their ammunition, their are two settings in MTW. The first and default setting gives them a limited number of fires while the option does exist to give them unlimited ammunition.
One common use is to put ranged unit at the head of an army, usually defending, to inflict some casualties on approaching enemy units. This will slightly damage the moral of units and weaken their numbers before they come into direct contact with other units. Usually these archers or other missile units will be recalled behind the safety of heavier troops before the enemy comes into melee range.
Ranged units behind an army's own lines can also be used to fire over the frontline into enemy reinforcements waiting in the wings if they are too close or as those reinforcements approach the frontline.
It is usually not wise to have ranged units fire directly into the frontline, as their missiles will also inflict heavy casualites on friendly units.
Mounted archers are particularly effective skirmishers.
From the high ground, these units have particularly devastating range.
Infantry units are usually the backbone of an army. Infantry are usually placed at or in the middle of an army to make sure it has a strong center. These troops are not particularly maneuverable, nor do they possess projectile weaponry. However, most exhibit good or adequate defensive capabilities, and any army without infantry is likely to fail when in contact with an army that does. These troops are often the placeholders of a battle, holding position and possibly doing some light maneuvering while calavalry do heavy maneuvering.
One view of battlefield strategy is that these troops represent core operations, and that cavalary and ranged units are used as support. In this school of thought, it is the effective use of the peripheral cavalary and missile troops that ensures victory, but a lack of infantry ensures a loss.
Spearman have long pointed weapons that allow the first few rows (rather than just the first row) of a unit of spearmen to attack simultaneously. The long spears carried by spearman make them adroit defenders, especially against cavalry. They are, in fact, the only unit that can stand up to heavy cavalary, one of the most feared units on the battlefield.
Spikeman have even longer pointed weapons that allow the more rows (rather than just the first row) to attack simultaneously. The long pikes carried by pikeman make them adroit defenders, thougt they form a thinner front.
Swordsmen genrally possess high quality armor and weapons which makes swordsmen very well-rounded soldiers with no large weaknesses that are suitable for most situations that arise in battle. Their above average morale makes them favorable front line troops.
Swordsmen are lighter than spearman which provides them with more maneuverability. Because of this they exhibit both fair attack and defense and stand up well against spearman.
Despite their extra maneuverability, swordsmen are still not fast enough to successfully chase fast skirmishers, and are not too light to stand up against heavy cavalry (especially when in a shallow formation).
Their professionalism also makes them vastly superior to militia and peasants.
Axemen can be seen as a subclass of swordmen. They genrally possess less armor than their counterparts. On the othor hand, their weapons is often stronger and has a bonus agaist amoured enemys. This makes axfemen very offensive-oriented soldiers that will often die just as fast as they kill in direkt combat.