The lowest level you can adventure here is 20, and the highest available Dungeon here is 40.
This is the same map as the ring of rooms surrounding the 4 green crystals in Silent Cloister main Hall.
Each player will be locked in a room with a boss. Clear the room to open the door, then click the Protection Stones in the main room to open the other doors and help your teammates. Additional bosses, and ninjas, will spawn. Keep clearing till chest unlocks.