Mercenaries

Mercenaries are a special class of unit that cost gold, but do not require local resources such as wood or recruits. They can be hired from cities that have the Mercenary Post attribute gained by an applicable building or general. The mercenary unit types that are available in a city are randomly chosen based on the native faction of the city. Once a unit is hired, a new unit will become available after a set amount of time.

Unlike normal units, mercenaries represent pre-existing combat units who have already been trained and may already have officers. This means they start immediately with a full force, but do not gain further experience and cannot replace men who are lost in battle. Mercenary units do not have a home city and do not automatically rebuild if they are defeated in combat. Mercenaries can be disbanded at any time if they are no longer needed or if they have been weakened to the point they are no longer effective

Strategy
Mercenary posts allow you to access units from other faction groups for a higher price and with a bit of veterancy added. It's good if you've got the income to spare and need a unit in a pinch. However, the selection of available mercenaries is random and partly dependent on your faction group. They cannot repair or recruit after being hired and should be dismissed when they are no longer effective.

Alternative
An alternative to recruiting mercenaries would be to capture a city of a different faction group. From there, the units of that specific faction can be recruited even without a mercenary post (as long as the city is not assimilated).