After a Unit has been trained it can also be leveled. Levels can be unlocked through Technologies.
The max level for a Unit is level 10. For each level your Unit gains you can apply a Promotion (listed below). These promotions are used to make your Unit stronger and give it special abilities.
Every Unit has a preferable Strategic Goodie. If these Goodies are not used then the cost will be increased.
You require technologies to level your Units to a higher level. You can upgrade it with 1 level if you have any of these techs: Military Tradition, Warfare, Conscription, Urbanization, Energy Study, Guarrilla, Global War, Military Corps and Mystical Enhancement.
The cost of promoting your Unit scales up.
When using the Units Preferable Strategic Goodie the cost of upgrading a Unit is: Current Unit level^2 * 12
If you have the Cheaper Unit Policy the cost of Strategic Goodies goes down by 20%
If you are not using the Preferable Goodie then the cost gets increased by 50%
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.