You can access the hidden level in this dungeon on Level 6.

You just need to play the level as normal but double back (see first screenshot below) to where the saw blade starts

you can then walk through the edge of the level and into the secret hidden gem level. (see second screenshot below)

You can access the hidden level in this dungeon on Level 18

Just drop down the gap to left of red button where the torches are lit. (see screenshot below)

You can access the hidden level in this dungeon on Level 25

This one is probably the easiest to spot, just go up middle platforms and jump out the top of level. (see screenshot below)

You will find the hidden level in this dungeon on Level 36

Make your way to the switch and ignore the key. Flick switch to reverse the moving platforms. (first screenshot below)
Go back up the way you came to get to the first moving platform at top of level and stay on it.
The platform will take you through the left edge of the level – just below where you started (second screenshot below)

Once all gems have been found the final level door is unlocked.