There seems to be one golden and another key missing (silver or copper depending on which door you open on Floor 2), so you cannot open all the cells in the higher floors.

See the mysteries marked on the maps below in blue letters.

• Floor 5 currently has 1 unsolved mystery (might be a bug)
  
• Floor 8 current has 1 unsolved mystery (might be a bug)

There is a grid with a teleport effect that can be temporarily disabled to pass it. On that grid, there is a switch that can be pressed only while it is disabled, to open a nearby secret wall.

Make sure to keep the Wooden Mallet, and do not break it. You'll need one later on Floor 3 for a secret.

Secret Area accessed from Floor 2: You will be able to jump into a trapdoor hole in the beginning of Floor 2 to access it.

I'm not sure if there is a smarter way to solve the puzzle of the 9 pressure plates next to the save point statue, but I just put all sorts of loot, one on each and was able to get through. At the end of the corridor opened by this puzzle is a Giant Deer Man. He is very strong and sometimes attacks twice in quick succession, so watch your positioning and if you dont have enough space to dodge, use a shield.

You can jump into the trapdoor hole in the beginning of the level to access a secret chamber on floor 1.

Both the Rusty Key and Silver Key lead to the same location. I only used one, and wonder if the other I kept would be usable on the next floor. Could be something for speedrunners!

Secret Area accessed from Floor 3: You will be able to take a set of stairs to a secret area. In here, put a torch on each side of the middle wall to unlock the secret and loot.

On the pressure plate puzzle, if you put a Deer Antler specifically on every pressure plate, it opens a secret room.

There is a buddha-like statue on Floor 3 who is said to be a god who wielded a mallet. The Wooden Mallet from Floor 1 can be tossed at it to reveal a secret in the room.

If you're stuck and havent gotten through to the Northern pressure plate near the grated teleporter, consider the room a bit more.
Hint 1: You can't open the grates by any means.
Hint 2: Remember how you opened some doors via placing items through grates?
Solution: Toss an item into the teleporter. It'll land on the pressure plate, opening the door!

In the area with many trapdoors and a set of stairs leading down to Floor 2:
Hint: Use the pressure plates to unlock a path across.

In the area with the purple hand statue that shoots a projectile you can get access to another hand statue that shoots fire. Let it shoot until it destroys all the doors and eventually shoots a large Red Ogre enemy dead without you having to fight it.

Near the entrance to Floor, the Golden Key opens a room with a bird in it. It'll run away, so kill it quick! It drops 4 healing potions (2 Rough, to higher tier ones).

The note about the "supply exchange" is misleading; it's not a puzzle required to get through those doors nearby, just stand between the two doors then toss something on the pressure plate and quickly go inside to grab the Inventory Upgrade.

This area has at least two rooms with multiple enemies within them. Be careful as you advance!

In the trapdoor area, you'll have to toss bones/antlers/bricks at a variety of pressure plates to first activate a teleporter, and then toss items in it. This puzzle leads to an Inventory Upgrade.

The area that opens with a Silver Key that says "danger ahead" leads to the Royal Treasury. There is no danger inside, only a ton of loot.

There is a magic wand that can be seen in the top right corner of the map in an area accessible from Floor 5, but is unreachable. You'll have to figure out which trapdoor hole to jump into from the floor above.

The two locations where you should put the gun(s) on the floor where the "deer eyes meet" are marked with a pink X.

In the area at the start where you can activate the purple hand that shoots missiles, you can redirect the orbs onto their targets at the wall. This opens a secret room with a shield.

The trapdoor area at the start contains a trapdoor that directly lines up with the one square room on Floor 4 that contains the otherwise inaccessible magic wand.

In the bottom right part of the map there are statues of a goddess of music. Play the lute in front of the two statues of her and then in front of the elephant to open the hidden room with the bird that drops healing potions.

In the right side of the map there is a large room with a lot of enemies locked into a central cage. There is no way to fight them separately, so use the environment to your advantage and come well-prepared!

Alternatively, you can avoid fighting them all at the same time by pushing the button then pushing it again after a couple of seconds, closing the gates, this way you only release a few of them at a time.

In the top right section with many trapdoors, you'll have to toss something or shoot at a button on the wall while walking on the timed trapdoors to activate a gate hiding a Rusty Key.

See the blue letter A below on the map.

Right at the start of the level you'll see an arena with new enemy types: parasite heads. You will have to enter the various small pathways before you can approach them to find the items required to unlock the door.
On the leftmost altar, put the Parasite Head. On the middle, a light orb (you can find them earlier but the level contains one as well). In the right one, the Parasite Hand. This will open the door to the large arena.

After defeating the enemies, find the small switch on the wall on the Eastern side by a skeleton to proceed.

This level has two entry points from Floor 6. The North-Western entrance has a Healing Statue, a Storage and 4 doors that can be opened with 3 Silver Keys and 1 Golden Key to get:

• Staff of Royal (Golden Key)
  
• a Steel Hammer, a Shield of Light and an Iron Trident (Silver Key)
  
• 5 Rough Healing Potions (Silver Key)
  
• 1 Medium Quality Healing Potion and an Iron Helmet [Protection +15] (Silver Key)

If you've explored the previous levels well, you will probably have on hand a Silver Key and a Golden Key ready to use. You can later unlock the path between this and the rest of the area from the other side.

From the other entrance there is a treasury with 5 Golden Keys required to loot

• 4 black gun ammo (x32)
  
• 3 white gun ammo (x24)
  
• 3 Phoenix Potions
  
• an Inventory Upgrade, a Golden Shield, a Legendary Sword and a Royal Dagger
  
• 2 boxes of shotgun bullets (x12)

Solving the 9 pressure plates puzzle unlocks a Medium Quality Healing Potion, a Lantern and a set of stairs to a pocket area of Floor 8 with a Save Statue, Storage but seemingly no way to proceed due to teleporters blocking the area. (See Floor 8 for more information)

In the room surrounded by the goddess of music, play the lute on the tile that looks different than the rest.

To the West is a pathway downstairs to Floor 6. Before going down, toss an item down each hole then descend. It'll open the path to a Mystic Key. After collecting the key, press the button on the wall to retrieve your items.

In the North part, there is a puzzle on the wall:

"O ye who walk before. If you have it, put it down.
Gazer into the Darkness.
Stone of Love.
Crystals of the primeval"

Put the Statue of Smiling Face, Staff of Earth and Necklace of Flame Stone into the alcove next to where the text is on the wall to reveal three secret rooms.

In the corridor with trapdoors that do not open when the button is pressed, instead a teleporter appears, toss an object into the teleporter to activate the trapdoors.

Once the trapdoors are permanently active, activate the teleporter again and stand on the square where it appears to find a secret room.

In the Northern part of the map that is accessible from Floor 7 near the 12 pressure plates, half the room is covered with teleporters. Do not be deterred, stepping into the teleporters is part of the puzzle. Start with stepping into the one on the left from the Save Statue.

In a Southern-East area accessed from Floor 8, three Mythic Keys open three small rooms containing:

• Royal Spear, a Golden Shield, two Phoenix Potions and a Legendary Sword
  
• 2 Shotgun ammo and 4 black gun ammo
  
• Adventures of the Great Sage Izami (+400 XP)

The treasure chest in the South-Eastern corner of the map can be used to transform items:

• Silver Coin -> Silver Key
  
• Silver Chalice -> Silver Key
  
• Gold Coin -> Gold Key
  
• Gold Chalice -> Gold Key
  
• Rough Potion -> Medium Quality Potion
  
• Stone -> Deer Antler
  
• Deer Antler -> Bone
  
• Creepy Glowing Orb -> Creepy Glowing Golden Ball
  
• Creepy Glowing Golden Ball -> Creepy Glowing Red Ball
  
• Creepy Glowing Red Ball -> Creepy Glowing Orb
  
• Gemstone of Flame -> Jewel of Flame
  
• Gemstone of Wind -> Jewel of Wind
  
• Gemstone of Water -> Jewel of Water
  
• Torch -> Lantern

In the room of truth, there is a puzzle with a purple projectile hand, a circle on the wall, two more projectile hands and grates that can be operated with two pressure plates. Stepping on the middle pressure plate makes the hand throw a projectile. Your task is to hit the projectile at the circle to the right to it and you can do this by standing on the pressure plate, waiting a moment and just as the projectile is about to hit you, move to the left, attacking with a long weapon (like a Spear). If timed correctly, the projectile will be hit and redirected at the circle. It's not over yet however, you'll need to quickly go to the next pressure plate on the left to operate the first set of gates, then to the pressure plate on the right to operate the second set of gates. Refer to the video below for a visual demonstration: