At Impel Down, we see Luffy descending down the different levels of torture, and going deeper and deeper where the harsher tortures were. Indeed, at Impel Down, we see 6 different levels, with increasing severity of torture from 1 to 6.
Its structure resembled something similar to Purgatory, in Dante’s Divine Comedy. However, its patterns of torture represent Inferno more. Hence, it is a combination of both hells put together.
NOTE the similarity between Purgatory and Impel Down:
1) In Dante’s Divine Comedy, Dante sees how Purgatory was structured. It was arranged into 9 terraces (see pic above). Inferno however, was arranged into 9 concentric rings with increasing severity of torture. A 10th ring is also present in Inferno, which is located at the centre, where Satan was chained up.
Purgatory is indeed, similar in structure to the 6 levels of Impel Down. (Note that the sea of Paradise is at the top of Impel Down, like how Paradise is in Purgatory). Inferno was similar to the tortures of Impel Down.
2) One shocking similarity is that in level 5 of Impel Down, prisoners having bounties more than 100 million are locked up, with no protection from the cold. We also see winds present, facilitating the freezing of the condemned. In level 9 of Inferno, sinners who have committed treachery and betrayal are trapped in ice for all eternity. The deeper level 9 is, the worse the conditions, leaving sinners frozen, with the icy winds freezing their flesh.
3) In Dante’s Divine Comedy, level 8 of Inferno were divided into 9 “bolgia’s”. At this level, counterfeiters and those who committed fraud were condemned, subjected into boiling lava for all eternity. It was named as the “heat hell”. Similarly, level 4 of Impel Down had boiling lava too, in which the condemned were at times, mercilessly thrown there.
4) Both One Piece and the Divine Comedy had icy conditions as being more severe than the heat hell. At Impel Down, Ice was at level 5 and heat was at level 4. At Dante’s Inferno, Ice was at level 9, and heat was at level 8 of the ring.
5) Both Dante and Luffy talked to the condemned, and even recognized people they knew long before. Dante recognized a man who committed suicide after being wrong accused, and kicked the head of Bocca degli Abati, a traitor of Florence, where he lived. Luffy recognized Buggy, Mr 2 and Mr 3. He even scolded Crocodile for his past actions against Vivi.
6) Both Dante’s Inferno and Impel Down had beasts that savagely devoured and gave living hell to the condemned. At Impel Down, we saw the mythological creature Minotaurus. We also see beasts like the Blugori, and Sadi’s subordinates. At Inferno, we see the mythological creatures, Harpy – which devoured those who committed suicide; the Cerberus, which guarded the gluttonous; and the Minotaurs, which ate those who committed Violence.
7) The well in Crimson hell used by Luffy and Buggy to fall to level 4 in Impel Down is similar to the well Virgil and Dante used to descend to level 9 of Inferno. Only difference was that the giants aided Virgil and Dante’s fall.
Of course, Oda would add variations using his own originality and creativity, which is why we see the “Crimson hell” as among the punishments used in Impel Down. However, the concepts inspired from the Divine Comedy are still there in Impel Down.