Units are the constituent parts of your army and are divided into classes that broadly define their role on the battlefield. They are recruited directly to your general, either in the field or when garrisoning a settlement. This is known as mustering. The only troops displayed are those troop types available in the region in which he is currently standing, i.e. it is not possible to draw in recruits from other regions. Select your general and then the recruit units button. Left click on a unit card to add it to the recruitment queue. When ready, units will automatically appear in the army of the mustering general.
Following recruitment of a unit or construction of a ship, funds are deducted from your treasury at the beginning of each turn to pay for the supplies and wages of the troops and crew. This is known as upkeep. Notice that your regular upkeep costs are detailed on the trade & finance panel and the financial summary.
Mercenaries are troops for hire. In return for the skill and experience of professional soldiers, a general must pay more to make them a part of his army, and keep them there, than he would a normal citizen or levied soldier. Like regular troops, they are sent directly to your general once hired, but, unlike them, mercenaries do not have to go through the multi-turn recruitment process first. This means they are available for battle instantly, making them ideal for tipping the balance of power in your favour. To reflect the expense of using mercenaries to fight in the ancient world, their upkeep is equal to their recruitment cost. This makes it imperative that they are discharged as soon as their work is done, lest their presence on the payroll bankrupts your campaign before your objectives have been achieved.