Creature in the Well
P.F.s Thinking Cap A.K.A 215,000 energy door.
If you don't drop through the hole in the floor and spend the time to gain enough energy in the first section of the mountain you can open the super expensive door in the timed room. Going through it and solving a couple other puzzles earns you a cosmetic that that also gives you the "Dual-Swords" ability and seems required to 100% this game. (I beat it with all the other collectables on a separate file and only had 99%.) There doesnt seem to be a way to get back on top if the mountain, so if you want this you'll have to do it before dropping through the hole into the main town.
Contributed By: Tlaloc.