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Finding the fourth crown
I finally figured this out years and years later when playing this game again. So I'm assuming that there are many others do not know how to find the forth crown.
To reach the deepest, bottomest, lowest dungeons of the game, go to the place near the bottom left, a place with colorful bad guys, and find the shop that sells the pick-axe for the girl to use to break blocks, and also sells the white armor. Now go all the way to the right of this area without climbinb the ladders. In the lower left of this area, between the two ladders, there is a secret hole in the floor you can open by touching. But I never noticed it because I always used the closer ladder to get out of this place.
Down below this hole, there are long hallways with almost endless rows of key doors. You'll require the magic key, which looks sorta like a horn, and it opens all doors without using up your keys.