This building allows you to train new special Units which can be unlocked after you brought the Fairies back to life in Chapter VII. The building levels decrease the Training Time for new Units.
In the Mercenary Camp you can produce five types of units: the Blossom Mage, the Drone Rider, the Ranger, the Vallorian Guard and the Fainéant Frog. These units came to your city with different guest races. This building shares its queue with the Barracks and the Training Grounds.
You are able to build the Mercenary Camp after unlocking it in the Research Tree and once you have the correspondent resources. It requires a street connection to the Main Hall, it can be sold, rebuilt and upgraded.
Each upgrade of the Mercenary Camp will reduce the Training Time.
Training your Units will cost you Supplies and time. The more Units are trained in a training slot at one time, the more resources are needed to start training and the longer that training will take to complete. Of course you can always finish the training instantly by paying Premium.
As for the Barracks, the amount of Units you can train depends on how many Armories you own and the weight of a single Unit. The squad size is also increased by the matching "Squad Size Upgrade" researches you can find in the tech tree.
Mercenary Camp overview