update
You can enter Stage 4 "Dungeon"

In each stage, you can collect a total of 3 gems and open the locked door
But this gem will be destroyed upon death
You have to collect three gems without dying
(After it's already opened, you won't lose your gem even if you die)
Also, when you re-enter the "Dungeon" after entering the "Dungeon", you must collect the gems again

bugfix
Thank you -
Changed files in this update