Mercenary Fortress is a location from Fire Emblem Awakening and it is the stronghold of the Sage Nelson, a member of the Grimleal and a fortress where Nelson's men uses to keep the stolen items from the village where his men taken.
There are two chest on this map that you can loot. One is up near the northeast corner and contains a Levin Sword. The other is near the center of the map on the west side, and contains a Bullion(M). A single Master Key is provided by an enemy during Paralogue 10, so you will need to either bring along a unit with the Locktouch skill, or extra keys of your own if you wish to open both chests.