There are 7 different branches of arms in the game, which all have various strengths and weaknesses.
Light Infantry is best deployed on rugged terrain, such as forests and in harsh weather. They don’t rely on tight formations, and their simple equipment means they don’t suffer heavy penalties under difficult circumstances. They are vulnerable to Archers and they can’t go toe-to-toe against better equipped opponents on open ground.
Heavy Infantry benefits from specialized formations, and quality equipment. These factors enable them to be very effective on open ground, both on the offense and defense. However, any terrain that prevents the use of tight formations will severely impede their prowess, especially if the ground is rugged, where heavy equipment is a liability. Thus, sending Heavy Infantry into a forest is dangerous. On the other hand, heavy infantry is well protect against arrows, even more so if they have large shields.
Spearmen are specialized fighters, best deployed against Cavalry. They are also effective against Flying Units and Giants. When pitted against foot soldiers, they are about as good as light infantry.
Archers are obviously meant for ranged combat, capable of decimating Light Infantry and Spearmen on open ground. They are the only unit type that is able to directly engage Flying Units (besides other flyers). Definitely not meant for melee combat, they must be kept away from enemy units.
Cavalry units – both Light and Heavy Cavalry – are invaluable for two reasons. First, their mobility enables them to capture key locations of the battlefield or flank the enemy lines and threaten vulnerable units. Second they are able to charge through infantry, trampling them and thus causing devastating losses without actual melee. Heavy Cavalry is especially good at this.
Flying Units play a very special role. Since they can fly over both obstacles and other units, they are able to strike anywhere on the battlefield. The only units that can directly engage them are Archers and other Flying Units, but that doesn’t mean ground units are completely defenceless against them. When flyers dive to attack, ground units will automatically attempt to strike back, although, with the exception of Spearmen, not very effectively.
Special units are usually monsters or other mystical units, who have special strengths and weaknesses. They fall into different subcategories like Giants, Wolves etc.