The game has 5 basic types of units: The spear unit, sabre unit, hammer unit, archer unit, and magic unit. The heroes are classified into these 5 categories as well, eg, your starting hero, Cao Cao, is a sabre unit.
Below is an overview of each type of unit:
1. Spear Unit
This is probably your main killer units. They are strong, and cause damages to multiple units at a time. The major disadvantage is they can only fire in a straight line, and thus positioning them is very important. They can be rendered completely useless in certain maps.
2. Sabre Unit
The damage output is similar to the spear unit, but they have only a small sphere of control, and they don’t hit multiple units like the spear unit. Thus they are only useful in corners and areas where you cannot place a spear unit effectively. I hardly use them in my games as they are completely overshadowed by the spear unit.
3. Hammer Unit
The damage output is low but at level 3, they can stun enemies, allowing you a few more seconds to deal damage to the enemies which is invaluable in higher difficulty settings.
4. Archer Unit
They are useful only in maps with flying enemies.
5. Magic Unit
Together with the Archer unit, they are the 2 unit types that can hit flying enemies. Other functions of a magic unit is to expose invisible units as well as collecting the dead enemies’ souls and converting them to energy for the heroes special skill.