After a Unit has been trained it can also be leveled. All eras from classical to modern provide +2 levels. Meaning your unit will be locked at a maximum level based on the era your civ is in.
The max level for a Unit is level 10. For each level, the Unit gains you can apply a Promotion (listed below). These promotions are used to strengthen your unit and give it special abilities.
The cost of promoting your Unit scales up.
The Hammer cost of promoting a Unit is 3000 * (1.7)^Current Unit level.
The Growth cost of promoting a Unit is 3000 * (1.7)^Current Unit level.