The Military Training Camp can be unlocked after the Warriors give 5 Moblin Trophies, 50 Lizalfos Trophies, and 200 Bokoblin Trophies to complete the Quest "Entrance Test". By talking to Rubeo, one of its workers, the Military Training Camp can be used to level up all of the Warriors for a certain amount of Rupees. As the Warrior's level gets higher, so does the fee.
Warriors can only be leveled up to the highest-leveled Warrior. For example, if Link has the highest level at level 30, then all the other Warriors can only be leveled up to level 30.