It's a wooden platform elevated on a rock. The tatami is in the center of a water area surrounding by a traditional Japanese grey and green building, we can see a giant orange Torii above the fighters.
Multiple spears are used as a barrier between the platform and the water.
On the water, there is various size of rocks and, also, some mysterious grey masked men observing the fighters. One of them, look at the match on the top of the biggest rock, he has a red mask who differs him from the others spectators.