The most basic unit and the first to be available upon completion of Barracks. They are usually armed with swords and travel on foot. Footmen have an advantage over Pikemen, but a very big disadvantage when facing Cavaliers. They are very versatile all-around units.
A more advanced infantry unit equipped with spears or halberds. They are recruited in the Barracks, like Footmen. Pikemen have more Attack than Footmen, but less defense. They specialize in taking down Cavaliers but are easily defeated by Footmen.
Ranged units with the ability to attack from long distances. Archers have a high attack against Cavaliers, though no defense and in a toe to toe battle with same level and same number of Pikemen, Archers will win every time. They are defensless against Footmen and against Cav... but certainly not against pikes. Not available until the Dark Age.
The combination of man and animal gives these units speed and power over all foot units except Pikemen, who have no trouble knocking Cavaliers off their steeds with spears and halberds. Cavaliers take up 2 spaces, while Pikemen and Footmen only take up 1. They also have a lot of Attack. Cavaliers can be built starting in the Feudal Age.
These artillery weapons are very powerful and long-range, but their size makes them very slow, not only on in-battle speed but also their attack speed. so guard them well. Ballista take up 3 spaces in population. When attacked from close range, Ballista can't attack back. However, all that firepower isn't easy to come by, so you'll have to be in Castle Age to construct them.
These are pretty much the most powerful units in the game since they aren't accessible until the Imperial Age is reached and a LOT of research is completed. Each civilization has two unique special units, one hand-to-hand and one ranged.