Accessing the Debug Room will reset/change your max HP, max SP, and max armor. There is no real benefit for accessing the room other than because reasons, so make sure to back up your saves first!

To back up your saves, go to the game's install location (easily accessed by right-clicking on the library entry > Manage > Browse local files. Once there you will see a number of "save.ini" files. Simply copy these files somewhere else, or archive them in a ZIP, RAR, etc.

To restore your saves, copy your backups back to the install folder (or unpack your archive) and overwrite.

ADVANCED USERS: You'll quickly notice the saves are unencrypted, plain-text INIs with (relatively) clearly marked values. Have fun.

First and foremost, you must have progressed far enough in the game to have obtained the hook. From there, access is easy.

Travel to the Forest Dungeon and head south one screen.

Immediately to the left, you will see a path of grass.










Follow it to the water's edge, face north, and then hook. You will be pulled off screen.
Here, things get a little tricky. Walking around, you'll hear that there are items sitting around that you can pick up, and if you walk right you can end up inside the trees on land. That's not why we're here (though I am curious what else might be out there).
After hooking across, walk straight left for a little bit, then south again so that you're standing on water. Then walk to the top-left corner of the lake. From here, continue walking left until the screen transitions to the cave.

This appears to be a test room for switch states and the charm ability.




The first room's switch responds to the player rather than the block.









The second room properly responds to the block.










And the final room has a charmable, but unreachable, slime.





At first thought, one might believe that's all. But wait! There's more! Exit the cave the way you came in.

Here's the meat and potatoes of the debug room. The teleporter orb is a inert, likely a sprite test for the orb, and cannot be used to escape, but the teleporter pad immediately to the right of the cave leads back to the succubus's home while the teleporter north of the pots leads back to Assen. Walking back into the cave instead takes you to the next area of the debug room, but more on that later. The chests contain modest amounts of money, and there is a key deeper in the area if you didn't bring one. Also scattered about are pickaxes, pots, grass, breakable rocks, unbreakable rocks, SP pickups and hook posts; there are no enemies here. Past the waterfall to the left is another cave leading to the second half of the debug room.

This area contains active copies of most/all normal enemies in the game and nothing else; if you take damage, retreating to the previous area will reset your HP and armor. A second cave door to the right leads back to the first entrance to outside area.

As previously stated, one may leave via the teleporter pads. Remember, however, that this area screws with your stats. It's best to just exit the game at this point. If you weren't thinking, teleported out, then saved, I hope for your own sake you read the warning at the start of this guide.

Enjoy screwing around with the debug area! If there's anything I've missed, feel free to let me know in the comments. For example, there appears to be more to the cave, but I've yet to have figured out if it can be accessed.