Here is the full list of all 25 Mystery Castle achievements.
Open a door with a magical key.
Teleport from one place to another.
Transform yourself into a bat, frog or troll.
Possess a golem or a ghost.
Drink a potion.
Slide over ice.
Make a golem push a box.
Find a hidden path.
Push down a floor switch.
Detonate a bomb.
Push 250 wooden boxes, ice blocks or metal crates into the abyss.
Open 100 doors.
Visit 5 different castles.
Walk over 500 cracked floor tiles.
Solve every room!
Defeat the Monster Baron in the Fortress Underground.
Defeat the Monster Baron in the Frozen Citadel.
Defeat the Monster Baron in the Enchanted Tower.
Defeat the Monster Baron in The Mystical Pagoda.
Defeat the Monster Baron in The Tomb of Fear.
Get caught by a golem.
Get caught by a ghost.
Burn to death on lava.
Get turned to stone by an Eye Spy.
Fall into the bottomless abyss.