Impel Down was a prison that the World Government used in order to hold the most dangerous criminals of the One Piece universe. It was once under the command of Warden Magellan and was comprised of six levels, with the lowest tiers keeping the deadliest characters.
As Luffy and his allies have demonstrated, it is possible to escape Impel Down despite the many precautions the marines have taken. Through identifying the anime characters who could pull off such a daring getaway and those who couldn't, we can better approximate how impervious the fortress actually is. Each escapee will begin their scenario out of their cell and unhandcuffed on floor six.
10 FAIL: Overhaul Has Abysmal Mobility & Couldn't Hurt Magellan (My Hero Academia)
Overhaul is reasonably equipped to escape Impel Down. The quirk that shares his namesake allows him to obliterate any physical obstacle in his path and skewer dozens of lesser guards. Further, he can heal from whatever injuries are inflicted against him.
However, Chisaki has average mobility and could not meaningfully damage Magellan on account of his venom-like physiology and Logia devil fruit. After the warden inevitably catches up with the Yakuza boss, he's as good as captured.
9 ESCAPE: Nagato's Almighty Push Could Sink Impel Down With A Single Cast (Naruto)
Pain's most infamous ability was the "Almighty Push," a shockwave powerful enough to devastate the entirety of Konoha in a single blast. His mobility would be facilitated by one of the dire beasts at his command, allowing him to outrun his pursuers should he choose not to defeat them in a direct engagement.
Once he'd reached the foot of Impel Down, he would collapse the fortress on his pursuers before flying away. The resulting carnage would disorient the marines too significantly to retaliate in time.
8 FAIL: Sloth's Philosopher Stone Is Finite & His Laziness Would Get Him Caught (Fullmetal Alchemist)
Sloth had the benefit of strength and speed, capable of reaching tremendous velocities when fully exerting himself. However, he is naturally lethargic and goes about any task he is assigned to at a torpid pace - including fighting Alexander Armstrong.
Since he was slowed down nearly to a halt by a dozen soldiers from Fort Briggs, it follows that the small legion of troops stationed at Impel Down should be more than sufficient in staunching the homunculus' escape until Sadi and her beasts arrive to detain him again.
7 ESCAPE: Killua Has Automated Defenses & Unparalleled Speed (Hunter X Hunter)
Killua's greatest strength is not only his unparalleled alacrity when in his "Godspeed" mode but also his ability to control his momentum. As fast as lightning itself and with automated combat reflexes, no one in Impel Down - warden or prisoner alike - would be able to catch up with him.
The only challenge he would face is when he actually reached the foot of the fortress and needed a ship. However, considering that the marines are usually docked close to the prison itself, he would be able to spring from a wall and land safely aboard a navy vessel.
6 FAIL: Maki Needs Flames In Order To Optimize Her Fighting Skills (Fire Force)
Although Maki was a decent fighter on her own merits (capable of defeating Shinra and Arthur without her ignition ability), her primary assets were her Sputter creations. They assisted her in battle and supplemented her already formidable martial strength.
As a second-generation fire user, she would need to reach level four to have any material she could work with. By then, the hundreds of wardens and feral wolves in the area would have prevented her escape.
5 ESCAPE: Pet Shop's High Mobility & Ice Would Make Him Dangerous To Pursue (JoJo's Bizarre Adventure)
Pet Shop was a hawk that guarded DIO's manor and an unlikely Stand user. He could unleash jettisons of ice in the blink of an eye large enough to obliterate cars and decapitate limbs.
Given his avian physiology and excellent navigational skills that were proven through his battle against Iggy, it would be impossible for almost any of Impel's jailers to reach him. Once he ascended to the foot of the citadel, he wouldn't need a marine battleship to make his getaway.
4 FAIL: Renji Has Low Mobility & A Limited Arsenal (Bleach)
Renji's Zanpakuto was a giant, snake-like creature that could ravage whatever he willed it toward. Despite its potential to devastate Impel's standard wardens and clear a path to the first floor of the prison, his low mobility and durability would pose a serious issue.
He was gravely affected by Mayuri's poisons during his battle against Szayel Aporro Granz. Subsequently, it follows that Magellan's venom would have a similarly devastating effect once he caught up with the hero.
3 ESCAPE: Whitebeard Could Shrug Off Any Attacks Made Against Him With Ease & Send All Challengers Flying (One Piece)
Whitebeard was once regarded as the strongest pirate alive and with good reason. He was powerful enough to survive hundreds of attacks (including being impaled through the chest) and was strong enough to collapse most of Marineford with his devil fruit ability.
Additionally, his Haki would equip him to defeat Magellan, as proven through his victory over Akainu. To make his escape, he would simply usher a tidal wave and force a navy battleship to lurch his way.
2 FAIL: Gowther's Magic Is Oriented Toward Precision Over Destruction (Seven Deadly Sins)
Capable of erasing his subject's memories through his arrows, Gowther may be one of the only Sins who would stand a chance of defeating Magellan by himself. His doll-like physiology is especially conducive to his success he wouldn't be as affected by poison.
However, Gowther's speed and strength are subpar; his attacks also lack the spread necessary to defeat hundreds of opponents at once. Subsequently, he'd soon find himself overwhelmed by soldiers regardless of the powerful adversaries he managed to topple.
1 ESCAPE: Armin Could Destroy Impel Down & Everyone In It By Transforming Into The Colossal (Attack On Titan)
As the Colossal Titan, Armin was capable of unleashing a devastating explosion through his initial transformation. The resulting carnage would collapse Impel Down and kill everyone inside, whether from being caught in the immediate detonation, crushed by rubble, or from exposure to the sea kings surrounding the prison.
In order to procure his escape, he would only need to eradicate the remaining marine battleships similarly to what he did in the attack on Marley, sparing one and turning it over to deposit what soldiers were aboard. Once the boat had been cleared of threats, he could sail away on it safely after destroying the gate sealing Impel Down, if necessary.