Alternative floors

[Cupcake]

I just unlocked them and i simply don't understand what are they for? do they give something special? or is it just too kill different bosses when you're bored and feel cocky?

[phantom dier WWWWWWWWWWWWWWWWWWW]

do you mean the alt path or do you mean stuff like burning basement / flooded caves?

alt path is how you access a different final boss and ending, and an unlock for each character for beating the boss. it's also how you unlock the true final boss and true ending

[Cupcake]

after i kill a boss there's this new door that i need a key for. Everything there shoots water and after i kill a boss there i have another door i can open with bombs. What do they do? Do i need to enter both in order to fight that final boss? Also what if i go to only one?

[steven]

It's a serie of alternative floors with different enemies and bosses leading to a final boss, giving you a mark for your post it and some unlocks for each characters.
i put some spoilers tags in my answer, look at it or skip it if you want to discover it yourself.

-In Cellar / Basement / Burning Basement 1, after beating the boss, a key door will spawn leading to Downpour / Dross 1.
The trap door of Downpour / Dross 1 will lead you to to Downpour / Dross 2.

- In Downpour / Dross 2, after beating the boss, a bombable door needing 2 bombs to be open will spawn, leading to Ashpit / Mines 1.
The trap door of Ashpit / Mines 1 will lead you to to Ashpit / Mines 2

- In Ashpit / Mines 2, after beating the boss, a door requiring 2 full hearts will spawn, leading to Mausoleum / Gehenna 1.
The trap door of Mausoleum / Gehenna 1 will lead you to to Mausoleum / Gehenna 2

- In Mausoleum / Gehenna 2, you will fight a Harder Mom, and a door requiring a special Knife will spawn.

To obtain this knife, you will need to do some special actions in the previous floors, giving you 2 pieces.

WARNING : in the 3 spoiler tags below, i indicate how to obtain the 2 pieces of the knife ! Skip this part if you want to discover or read the spoils

*Knife piece 1 : Located in Downpour / Dross 2, throw yourself in a white flame spawning randomly in a room and you will be the Lost.
Go in the mirror located in another room and you will be in a special stage.
The Knife piece 1 will be in the treasure room.

*Knife piece 2 : Located in Ashepit / Mines 2, find 3 yellow buttons in the floor, press them and go to a special double sized room with a minecart and some tracks. Take the minecart, you will break the door at the other side of the room and go in a special stage, naked (all items you had will be taken for the duration of the stage and will be given back at the end).
The knife piece 2 will be at the end of the maze.

Knife piece 1 and 2 will fuse in a familiar that can be throw at the door in the boss room of Mausoleum / Gehenna 2

END OF WARNING

The door broken with the knife, you will fight a harder Mom's heart.
Go back to the previous room and the trap door will lead you to Corpse 1.
The trap door of Corpse 1 will lead you to to Corpse 2, the end of your road.

- In Corpse 2, you will fight Mother, the final boss of this path, unlocking items for your characters, a mark on the post it and an ending.
It ends your run if you are on Repentance, or you can continue to the Void in Repentance +

All this alternate floors have 2 items in the treasure room instead of 1 of the regular floors. One is hidden and one is shown. You can only pick one item.

You can access the alternate floors or going back to regular floors at any time.
You travel from :

- "Regular floor 1" --> "Alternate floor 1" or "Regular floor 2" --> "Alternate floor 2" (and not "Regular floor 1" --> "Alternate floor 2", you don't jump a number like "Regular floor 1" --> "Regular floor 2")

- "Alternate floor 1" --> "Regular floor 2" or "Alternate floor 2" --> "Regular floor 1 of the next chapter"

[Cardboard]

Originally posted by steven:

It's a serie of alternative floors with different enemies and bosses leading to a final boss, giving you a mark for your post it and some unlocks for each characters.
i put some spoilers tags in my answer, look at it or skip it if you want to discover it yourself.

-In Cellar / Basement / Burning Basement 1, after beating the boss, a key door will spawn leading to Downpour / Dross 1.
The trap door of Downpour / Dross 1 will lead you to to Downpour / Dross 2.

- In Downpour / Dross 2, after beating the boss, a bombable door needing 2 bombs to be open will spawn, leading to Ashpit / Mines 1.
The trap door of Ashpit / Mines 1 will lead you to to Ashpit / Mines 2

- In Ashpit / Mines 2, after beating the boss, a door requiring 2 full hearts will spawn, leading to Mausoleum / Gehenna 1.
The trap door of Mausoleum / Gehenna 1 will lead you to to Mausoleum / Gehenna 2

- In Mausoleum / Gehenna 2, you will fight a Harder Mom, and a door requiring a special Knife will spawn.

To obtain this knife, you will need to do some special actions in the previous floors, giving you 2 pieces.

WARNING : in the 3 spoiler tags below, i indicate how to obtain the 2 pieces of the knife ! Skip this part if you want to discover or read the spoils

*Knife piece 1 : Located in Downpour / Dross 2, throw yourself in a white flame spawning randomly in a room and you will be the Lost.
Go in the mirror located in another room and you will be in a special stage.
The Knife piece 1 will be in the treasure room.

*Knife piece 2 : Located in Ashepit / Mines 2, find 3 yellow buttons in the floor, press them and go to a special double sized room with a minecart and some tracks. Take the minecart, you will break the door at the other side of the room and go in a special stage, naked (all items you had will be taken for the duration of the stage and will be given back at the end).
The knife piece 2 will be at the end of the maze.

Knife piece 1 and 2 will fuse in a familiar that can be throw at the door in the boss room of Mausoleum / Gehenna 2

END OF WARNING

The door broken with the knife, you will fight a harder Mom's heart.
Go back to the previous room and the trap door will lead you to Corpse 1.
The trap door of Corpse 1 will lead you to to Corpse 2, the end of your road.

- In Corpse 2, you will fight Mother, the final boss of this path, unlocking items for your characters, a mark on the post it and an ending.
It ends your run if you are on Repentance, or you can continue to the Void in Repentance +

wheres the key door at for the first step

[steven]

Originally posted by Cardboard:

wheres the key door at for the first step

It is said in my post, but i modified it multiple times so here the answer.
-In Cellar / Basement / Burning Basement 1, after beating the boss, a key doorwill spawn leading to Downpour / Dross 1.
The door spawn in the boss room