Mageslayer - Secret Levels
Each of the five worlds contains two secret levels, making ten in total. Aside from "Run the Gauntlet", secret levels are generally a flat maze themed around a single specific enemy type. You are not allowed to save while in a secret level. If you die while inside one, rather than the usual penalty, you are simply kicked back to the level you entered from and reset to the state you were in before you entered; the secret level entrance disappears. If you are victorious, on the other hand, you get to keep all the goodies you collected.
Run the Gauntlet
Accessed from Sewer Channels: Rat Swarm. After descending on the big elevator, go east to the platform that has a different floor color and check the eastern wall.
Maze of the Arborite
Accessed from Sewer Channels: Rat Lord's Lair. After descending the stairs, go west and check the dead end wall at the end of the big corridor. This level is subject to a bug which can ruin your game, so keep a save from before entering if you go here.
Barbarian Toad Haven
Accessed from The Stronghold of Fire and Ice: The walls of the Stronghold. From the platform surrounding the central tower, which has drawbridges on the other three sides, go to the east side and shoot the wall on the far side of the lava looking for a portion that explodes. Jump across the lava to get to the secret exit that is revealed.
Accessed from The Stronghold of Fire and Ice: Into the Depths. When you see a half-circle-shaped wall on the east side of a corridor, bump against it to make it spin and reveal the secret area.
Accessed from The Desert Keep: Movements of the Earth. After riding the platform across the lava to reach the key, look for a hard-to-see magic platform of red particles just to the west of the key platform. Ride this up to the secret exit. To get back from the secret platform, jump down to the south to land on the bridge where the locked door is.
A Genie's Harem
Accessed from The Desert Keep: Machinations of Doom. Go to the east side from the central lava pit where the exit is, and head to the room just north of there to break open part of the north wall with your attacks.
Accessed from The Temple: The Outer Gallery. This one is hidden in plain sight; when you get to the diamond-pattern platform over the acid pool, drop down into the southeast side of the pool and the exit is right there.
Lore Thrall's Dream
Accessed from The Temple: The Temple of the Spider. South from the pit with the healing items, across a pool of acid, you can find a metal wall; this is a secret locked door. When you get a key, bring it here. This level is subject to a bug which can ruin your game, so keep a save from before entering if you go here.
Accessed from The Tombs of the Lore Thane: World's End. When you get to the section that has platforms which flip over above a lava pit, check the south wall of the south-center alcove.
Accessed from The Tombs of the Lore Thane: Damnation's End. After you press the button past the lightning traps, check the northeast corner of the corridor the button was in.