The Crystal Lake is a building which passively grants experience to monsters inside the building. Up to a maximum of 4 monsters can be placed in this building, with each one gaining 150XP/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 Crystal Lake, press and hold on a monster, then drag it onto the structure. If a green arrow pointing downwards appear, it means that the building has vacancy and monsters can be dropped in it. If a red arrow pointing downwards appear, it means that the building is full. Alternatively, monsters can also be selected by highlighting the building and then placing the monster from the list. It is recommended that players lock the monsters in this building to prevent them from being used.