In each age, there are five different military buildings that produce the different units of that time. The units differ in their type: There are fast units, light and heavy units and units with short or long ranged attacks.
Fast units have a good range of movement, but they cannot take much damage. They are a great way to take out ranged units.
The light units are genuine all-rounders: they are fast, inflict good damage and can withstand a lot. They are best at intercepting enemy cavalry.
The heavy units are slow and heavily armored. They are formidable opponents when they make it into close combat; there they outmatch any other unit.
Short ranged units are fast and can attack from a distance, and their attack does significant damage. However, they cannot defend themselves and are easy prey for all units which manage to attack them in close combat.
Artillery units are lost in close-range combat. They are very slow and have a weak attack. Their big advantage is that they can cover almost the entire battle map with their attack and are able to selectively attack almost every unit.