After passing through the Teleport after the Crystal of Life, and then two more doors, there is a lever in a room to the left. Pulling it will open another room on the left, before the one with the lever.
After teleporting and going through the first two doors there is a small button on a southern wall, 2 blocks west of the lock/keyhole. This opens the Hidden room, which is found in the first room after the second door on this level (doesn't counts as secret in statistics).
After the first puzzle is an area with some pits. There is a button on the reverse wall of the pillar in the middle of the room which opens the middle of the pillar. Throw a rock across the pits onto the pressure plate to close the pits and access the secret.
In the room with two separated pits (down the corridor from the Crystal of Life), press the button on the wall before the first one. Quickly turn around to face the opposite direction and move three tiles left to cross over the pits before the trap doors reopen. Pressing the button on the wall above the second pit will reveal a Teleport which will take you to the secret.
# In the same room with pits, jump down into one of them. First, place a Rock on the pressure plate in the west end of the chamber. There is an area with pressure plates behind grates. Press the button on the wall nearby to disable the Teleport (only works if pressure plate to the west is weighed down) and place rocks on the two pressure plates to open one of the grates. The secret is at the back of the area.
In the room resembling the first puzzle (only with spiders), place the torches in the opposite pattern: torches on walls, none on the pillar. This will reveal a short passage with a lever and view of the Iron Door. Pull the lever to open the door, to which you can now return. Remember that to close the trap door, you must take the Rock off the pressure plate used in the pillars of light puzzle.
# On the wall of the room of Secret 7, there is a hidden button which reveals stairs down to the Slime Dungeon. On the far right side of the room is a button which reveals a room with the Green Gem. Place it in the mouth of the demon face near the stairs to reveal a large secret room.
In Time and Tide, after pressing the plate before the race across trap doors, turn to the right on the second trap door. There is a button on the wall that opens a wall just ahead with an alternative way, but also with trap doors.
After completing Trails of Thought and reaching the room full of pits and Crowerns, there is a button hidden in the small alcove. Press it and run into the pit area, crossing the closed trapped doors and pressing the button on the wall. The Teleport in the center of the room is gone and the trap door under it is open. Drop down and collect the Gear Key. Returning to the hub room, use the key to open the Iron Door which is located down a hallway to the NE. This is incredibly hard so be prepared for frustration.
In the Catacomb, there is a hidden button in the hall NW of the room with the 4 Undead Soldiers waiting behind a grated door. Press it to open the wall at the end of the suspicious dead-end hallway to the north. You will need to defeat 4 Undead Soldiers then press the button again and rush to enter in time as the door will automatically close.
In the Catacomb, after releasing and defeating the formation of Undead Soldiers behind the grated door, press the button in the area they were in. The grated door will close behind you and the wall in front of you will open.
Northeast of the room with 9 pits, there's a door with 2 buttons and a lever. The conditions to open the door are unusual: the button next to the grate must be pressed at least once to start the puzzle, then the switch must be flipped at least seven times. Once these are met, if the switch is set upward, the button next to the door will open it.
As the nearby Scroll indicates, to open the Iron Door on this level, you need to find where the Stone Dragons gaze, meaning you need to find where the gazes of the two dragons intersect. This is just 10 blocks west from the first dragon statue. Stand on that square for about half a minute until you actually hear the door opening, or rest there for a short while.
On the left wall where you find the Fire Bombs, there is a stone with a rune on it. Press it to open a wall. To pass the grate, put (don't throw it) an object in the teleport that appears when you walk on the pressure plate. When you will take the Orb of Radiance, 2 Giant Crabs will appear in the room behind you, even if you put something there. You will have to press the rune stone again to get out.
In the "Maze of Madness" room, where you pick up the Round Key, look at the wall to the right, there lies a hidden button. Press it to disable a teleporter. Make your way back around to the S side of the room, you should see the pillar along the south wall is gone. Go forward to find the Pendant.
In the "Walkabout" area north of the entrance, there are three hidden pressure plates. When you step on one, you'll hear a click. Seemingly they do nothing, but there is a trick. Walk around the entire hall clockwise, hitting all three plates, after two times around a Rock should appear. Go around again and an Undead Soldier will appear. Make one more full pass around the room and the Sack should appear.
Still in the Haunted Halls, north of the pressure plate, there is a stone pillar in the middle of the room. On its east side, there is an hidden button which opens a room on the south-east corner of the Halls. You might have to look for a little while.
# In the Mage's Entrance area, drop down into any of the trapdoors. Make your way through the tunnels and drop down the pit in the south. You will find 2 doors controlled by a pressure plate. Drop something on the pressure plate to open the first door, pick it up from the other side to open the second door.
# In the Mage's Entrance area, within the Fireball Hallway, there is a hidden button on the east wall, right after the pressure plate (closer to the head). Pressing it opens the wall behind you. There you will find another hidden button. Next, locate a third hidden button on the west wall, 2 tiles before the pressure plate. Pressing it opens the wall next to you, where you will find stairs leading to an area in Level 5 that was visible but inaccessible.
In the Mage's Entrance, turn north after completing the first challenge. You should see a small rune on the wall. Press it to open a secret passage leading to the Fighter's Challenge. You will need the Sword of Nex to enter the challenge proper. After you beat the Fighter's Challenge, turn your attention to the west wall. There is a hidden switch and room with treasure.
In the room south of the Crystal of Life, there are 2 pressure plates. Stand on the northern one until the nearby portcullis is completely open, then quickly step south and back north to open two secret rooms.
At the west section of the map, there is a seemingly unimportant area where you can find a Flail. There is a rune symbol on a nearby north wall. Pressing it opens a timed secret door on the passage east of you. You can find a scroll detailing this location somewhere earlier. In the secret room, there is a stairway leading to the Dismantler's Vault section of Level 8.
Access to Dismantler's Vault
To open the Iron Door near the entrance to Level 8, Intercept all the projectiles shot by the three demon heads. The pattern remains the same between attempts, but the projectiles constantly get faster. You may need to turn down the display quality to move between the tiles fast enough.
From the Dismantler's Vault, in the Lightning Conduit tunnel, the third room on the north side of the tunnel has 3 hidden buttons on its walls. Each reveals a pressure plate, upon which you must leave an item to open a secret area in the NE corner of the room.
At the very end of the Lightning Conduit tunnel, there is a hidden button on the north wall which opens a secret passage on the nearby south wall. The last room on the south side of the tunnel has a hidden button which makes accessing this secret possible without getting hit by the lightning.
Finally, entering The Vault itself in the Dismantler's Vault counts as a secret. Drop something on the pedestal where The Dismantler lies before picking it up to avoid the lightning traps. There is a switch on the wall behind The Dismantler, press it to open a secret door to get the Golden Dragon treasure.
After solving the teleportation puzzle in the Outer Sanctum, drop into one of the two pits. Behind a grated door there is a scroll with instructions on how to open the door: place a torch in each sconce, then stand on the tile where their shadows intersect. Throw one Rock north, and another east.
After unlocking the teleportation room, look around the north walls for a hidden button; pressing it will disable the Windgate and open a trapdoor beneath it. After dropping down, before taking any of the items, search the wall for another hidden button—the demon's head will fire lightning, but the button will seal it behind a wall.