Hidden Floor Main Door
This door cannot be destroyed even by a Ranker like Karaka and won't open no matter what he do. The door apparently seems big when seen from the outside, but turns out, it's nothing compared to when it was seen from the inside. From inside the Hidden Floor, the main door is so huge and was seen floating above the sky in flat-down horizontal position.
This door is heavily locked and can only be opened by using the Master Key. To be able to gain access to enter this "main door", you should obtain just one of the two master key. One key is currently in the possession of King Zahard and the other key is in the hand of Hell Train Conductor (Currently, the Master Key of Hell Train Conductor was stolen by Po Bidau Gustang and was given to Baam). Rankers cannot enter the Hidden Floor even if they have the Master Key. If Regulars can obtain the Master Key and open this "main door", it will allows them to ignore the third rule of Hidden Floor. However, they cannot save their data or record in the system.