In Warbirth, Mercenaries are most important units as they form a vast part of your unit roster. They may cost very ittle in the way of wealth at the beginning, or have powerful units that can turn the tide of war for you. Thus, use them with care where you can.
Unlike Chronicles where you must build an Outpost, these units are instead raised from various sites. Some Mercenaries require the use of a Fort. Others a House of Worship. But the most common building required to recruit them is obviously the Mercenary Camp, which is a unique build that can be built almost anywhere.