Officers

Experience
Units can be upgraded after they have gathered a minimum amount of experience points. Units will gain experience in battle and by being built in cities that have been upgraded themselves with special structures (training grounds). Experience points can be converted into unit upgrades ("officers") that make the unit better at a specific task. For each upgrade there are 4 levels. Gaining the 5th level allows the player to create a compagnion, i.e. a special unit that can be employed as general of an army or governour of a city. Mercenary units however do not gain any experience.

List of Upgrades
For every officer there are 4 levels, each with better effects. A level 4 officer may be upgraded to a general, which does not improve its stats but frees up the officer slot and allows it to be detached from the brigade and reassigned elsewhere.

Strategy
Once a unit is sufficiently upgraded, a new champion (i.e. a general) can be drawn from it, detached and moved on the map to either attach to another unit or become a governor in one of you cities. This provides the player with a great tool to increase his flexibility in empire management, particularly since champions come on horseback an can reach remote places quickly.

Most importantly, never delete units unless you absolutely have to. Experienced warriors are a godsend in this game, and can take on enemies that would crush them in any other situation. Cheaper than having an army twice the size in the late game to compensate for the greenness of your warriors.