The underground "swimming pool" of the castle. The water in the caverns are so clear, you can see down to the bottom and would never think it's several meters deep. The cave's upper part is shimmering as if you were looking up at the night sky, these, however, are gemstone, reflecting the magical lights which are floating all over the "labyrinth" of water.
The water is warm at all times and the cave also has a comfortable temperature.
This is where students and teacher can come if they want to practice their magical skills in duelling. It is not rare to see two of the Headmasters playing matches here as they like to challenge each other in duels.
There is a ward underneath the arena which makes sure all spells used here are none-lethal. However, students can still feel pain if their opponents use stronger spells. Usually, the only thing they can feel is a small or bigger electric jolt.
The arena is a big round room, with seats surrounding the actual fighting zone. People are welcome to watch matches and cheer for their supported duelist.