Maybe for challenge,or for pirates/revolutionary palyers to escape after death based on players' bounty
when get killed by marine npcs or players,they will respawn there
water fill the interval of each level(in case of door fruit and other df escape easily)and all covered by obsidian
Level 1
there is Haribariso(add new grass-like block) surround limbos,which can make prisoners passed by bleed
Level 2
Full of beasts on the road of jail(ravagers and mmnm's gorilla are ok)
Level 3
a wide desert and player in this level will gain hunger and slowness effect
Level 4
with a solid and structure and lava on the base of this level,so palyer in this level will get burnt
Level 5
covered by snow,give palyers frostbite effect and lose hp gradually,there will be angrily wolves chasing you
Level 5.5(none)
Level 6
marines(even captains)spawn here,all the roads are coverd by kairoseki(there will be marines spawn in other levels randomly but less)