The Inexplicable Door (also known as the 8-Bit Realm) is found in the The Distant Woods. It only appears if you have a continuum transfunctioner equipped, and is only accessible if you have 20 in your main stat. The pixellated monsters here all have 15 HP and don't drop any meat, but they are rich in pixels. All the monsters can drop up to three pixels. Wearing the 8-Bit Finery will make you deal more damage to monsters in this area.
If your main stat is too low:
You try to open the inexplicable door, but can't seem to muster up the strength and/or wisdom and/or panache to do so.
You wander off in search of more appropriate things to do.
(You must have at least 20 Main Stat to adventure here.)
Monster Level: 25 • Substat Gain: 6.25 • Moxie for No Hit*: 35
Monster Defense: 22
Hit Points: 15
Initiative: 60
Elemental Alignment: None
References
The monsters here all originate from games (more specifically, Super Mario Bros. and The Legend of Zelda) for the extremely popular 8-bit video game console, the NES.
The Inexplicable Door's name itself refers to earlier (mostly 8-Bit) games that had a door outdoors that you had to enter to complete the level. These doors weren't attached to any building, and were thus inexplicable.
The fact that you encounter Buzzy Beetles instead of Goombas after you ascend is a reference to the fact that that, after beating Super Mario Brothers, you play a harder set of stages where all the goombas turn into buzzy beetles.