The dungeon hosts a few secret portals that gain you access to special areas not part of the standard dungeon route! These special realms often give you something special in the shape of Friendlies, Items or Weapons. On top of that they will also give you a short cut to skipping one or two levels!
The Dark Woods entrance can be found in Dungeon 1-2. A strange statue covered with vines will be placed in a room, and it covers a locked dungeon door.
You can nudge the statue by dashing against it a couple of times, and it will vanish.
You’ll then need to use the Boss key to open the exit and enter the Dark Woods Realm (WARNING: so don’t use that boss key on the actual boss-door.. can only use it once)
The Dark Woods hosts King Stout as the end boss, and you can gain the Fairy crown by defeating him. That will then unlock the Fairy Friendlies who hoover around you and attack your nearest enemy. You can gain more of these Fairies by uncovering them in the various wooden room debris.
The Temple entrance can be found in Dungeon 3 (level 1, 2 or 3). The first monk you encounter will give you a quest, leave the room, and go back in. The monk will be gone but he forgot to close the portal to the Temple!
The Temple exit is guarded by Sandy, a big sand worm. Once you defeat her you gain the Crawler Crown – which unlocks the Crawler friendies who run behind you and use their little snapping claws to hurt enemies!