The door has the egg easter egg on the other side if you interact with the door twice and Ralsei is at the bottom of the fountain in the boss arena

I don't know anymore from there

There three purple "key" fragments:
1) found at the corner at the end of the path going away from the castle.

2) found at the bottom of the stairs leading to the fourth floor (you'll need to use no-clip to get it).

3) In a room at the end of the corridor on the fifth floor (its hidden behind second portrait on the bottom row going from the right)

Once you have all three key fragments, you can unlock the door in the "sub-basement" (where "Jevil" is), walk up to him and then make your way back to the lift (where you will see him "disappear" through the back of the lift).

(There are also pictures that change when you interact with them)

引用自 springjohnnyrogers：

Are there any secrets I should be aware about? I know that apparently one of the doors in the party rooms (I think) jumpscares you if you no-clip through it and there's also another door at the beginning of the map if you turn around (Which I'm a little worried to know what's on the other side)

Is there anything else I should know?

In respective order:
1) you don't need to no-clip through the door (in the "party" room) to get jumpscared (I checked).
2)The door at the end of the path just goes to an area with a tree in the centre of it (does that put your worrying to rest?).