The Three Functions Of The Coliseum
The coliseum Aroon thought of had to serve three functions at the same time. First it had to be spacious enough to contain and kill the monsters inside.
After all he wasn't fighting against four exhausted bosses, but eight. Four of them were healthy and perfectly intact. So the levers inside the coliseum worked as a killing machine to kill those injured in less than one minute.
The second function and the second most important advantage was to defend. Aroon knew the moment he would summon his artifact here, many would try to attack him and his kings.
Seeing things this way up ahead made him realize the importance of having strong defenses in his gigantic gear. And that defense couldn't rely upon the response of him or the other kings.
After all, the mission of killing four strong bosses, even if they were exhausted, was risky. He couldn't risk jeopardizing the entire process for acting on defense.