The Deathcropolis is a gladiatorial colosseum, located within an unnamed city on the planet Yarno.
In "Chapter 3", it was used by The Order of the Twelve to make sacrifices to the "Twelve Titans". All refugees who came to Yarno to seek a sanctuary where instead sentenced to fight in the Deathcropolis.
While the Order didn't want to have Mooncake participate in the fights, since they planned to give him to the Lord Commander, Mooncake still ended up in the arena when trying to escape his cell. He was forced to fight to other creatures, and killed them all. Then the Lord Commander himself arrived to take Mooncake, killing the last gladiator in the proces. Mooncake fought back against the Lord Commander, and was soon joined by Gary Goodspeed and Avocato (who had escaped the Lazarus Trap they had been send to). Together, they escaped Deathcropolis, while Lord Commander was distracted by the other monsters Gary released into the arena.