The prison has a tall gate and tall walls with barbed wires. The building is heavily monitored with cameras on different locations with large monitors for the guards to see. For more difficult prisoners like Speed-o'-Sound Sonic, they use a special restraining device designed by the Hero Association. The cells that hold the most difficult prisoners are behind a thick, barricaded door. Inside are the most difficult offenders, who can use their strength to pry open iron bars or can use everyday objects as weapons and poison. The inside of the walls are made of steel and there are no windows, and only Puri-Puri Prisoner can break through the walls.
The prisoners-turned-monsters seem to be only a few:
- A monster with a crab-shaped head with six crab legs hanging from his temples. He also obtained a tail that ends in a star-shaped tuft at the end.
- A simplistic round monster with sharp teeth and two muscular arms.
- A dinosaur-like monster similar to a dilophosaurus.
- A log-head wooden monster.
- A bug-like monster.
- A monster with a canine face and an extra eye on his forehead.
- A monster with an asymmetric grin, multiple scars on his face and two wings right above his ears.
- A squid-faced monster.
- A round-faced monster with multiple eyes and multiple tendrils on his head.
- A round-faced monster with big teeth and webbed tentacles below his chin.
- A horned monster with a sharp skull.
The real identity of the monsters has not been confirmed.
- ↑ 1.0 1.1 One-Punch Man OVA; Puri-Puri Prisoner and the Jailbreaking Prisoners
- ↑ One-Punch Man Omake; Prison
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