"A practice field made strong enough to endure the power of the Monsters. You can train Monsters to make them stronger."
The Practice Battle Field is a building which passively grants experience to monsters inside the building. Up to a maximum of 5 monsters can be placed in this building, with each one gaining 200 XP/hour. Monsters in this building cannot be stored in the Monster Storage, although they can be powered-up or evolved.
To put a monster in the Practice Battle Field, select the building; its slots will appear, showing you the monsters currently inside. Click on any slot to select from a menu which monsters you want. Alternatively, you can also press and hold on a monster, then drag it onto the building. If a green arrow pointing downwards appears, it means that the building has vacancy and monsters can be dropped in it. If a red arrow pointing downwards appears, it means that the building is full.
The Practice Battle Field is a crafted building. In order to craft it, the player must pay not only the crystals, but also the following crafting materials: 20 Solid Rock, 20 Solid Iron Ore, 100 Condensed Magic Crystal, and 5 Essence of Fire (High).