Sometimes you find these by doing something weird. Sometimes you gain access after doing everything else possible in a game. Whenever I learned of such levels, I would make a point to earn them, no matter the difficulty.
Sonic Adventure 2 sticks out the most in my mind because almost never got to see it. I almost destroyed my Gamecube because I thought it was broken. I also thought the three emblems I was missing were impossible to get. It took years of wisdom before I finally won and it’s a good thing that winning is its own reward because the extra level is not so fun.
Wario Land II has a great final level that’s horrific and bizarre. You have to use techniques that aren’t explicitly demanded anywhere else in the game.
This stage just feels good.
Please share your favorites. There are so many that I can’t think of right now.