Shikou, an underground prison named after four legendary sinners, will be added soon.
Look for the stone monuments that appear in "HARAE" when the conditions are met.
Each tower has a "netsuke" hidden inside.
If you escape with the netsuke, you can take it back with you.
Note: The netsuke itself has no special abilities.
You can see the netsuke you have collected in the "Netsuke" section of the "Achievements" page.