Quick tip for the people who are starting a new game like me (Didn't finish the game, but wanted to play differently).
There is a locked circular door in the lake area, floor 3, that needs two items to be placed.
1st "Defiled Shrine Maiden Statue": In the tutorial, avoid getting killed early, pick it up at end of tutorial.
2nd: On first floor, use the blasting bomb on the secondary broken wall (behind a impure seed statue, in a location with some canals). (Unless you can get a second bomb, I don't know if you can still break the first wall)
EDIT: Someone replied that there is a way to return to tutorial area, going through a path of leech traps in the west part of floor 1, in the location with red enemies, below the photographer escort quest location. This truly is the dark souls of threads.
EDIT2: I've also found a third breakable wall on the first floor, in the location directly northeast of the entrance. Don't know what's behind it, but I suspect you can get more blast bombs at some point?