The Mercenary Camp is a building in Warbirth which is capable of allowing you to recruit low-level mercenaries. These buildings unlike most others can be built anywhere on the map with dry open land available, so that you can then supplement your forces further with fresh runners. Mercenary camps will rot very quickly in the middle of enemy territory - but such is the price you pay for convenience.
Not all Mercenary units may be trained here. Some others, such as elite cavalry, must be raised only from the Fort. Others too require a House of Worship to be constructed. It is thus vital for you to know what units are trained where, so that you can then plan your strategies ahead of fighting.
Mercenary Camps can be unlocked only with Level 2 Military.