General Tactics
Weather
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, longbowman, crossbowman, 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 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/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.