If you intend to tackle Nightmare difficulty, follow the steps below.

To the left and to the right of the doorways that lead to the Episode selection slipgates are dark dead ends. If you stand next to the wall farthest to the left or right and look up, you should see a shootable button in each of these two hallways. There is a third button as well - it is behind the portal that takes you back to the difficulty selection room. Aim a little higher than your head and shoot it.

Once this is done, two small secret lifts will be active under the lava-filled hole in the ceiling (near the portal where the third button is). Ride them up and you will be teleported to a hidden chamber, where the Nightmare selection can be found.

There are 4 secrets here.

• Under the first elevator you come across, atop of the structure at the right of the starting zone. Trigger it and jump in the hole below to collect the Super Shotgun, shell and nail.
  
• After collecting the silver key, you have to use a small elevator leading to the boat pen. Jump in the slime under the elevator and quickly pick up the biosuit then follow the path to find a 100 Health and Lava Nails.
  
• After your second ride on boat, enter the building and shoot the button above the door. A Power Shield will appear in the small corner across the slime lake right in front of the door.
  
• Jump in the slime canal under the drawbridge to find a biosuit with rockets, nails and 3 health.

There are 4 secrets here.

• At the starting area, dive into one of the water holes and push the button on the wall of the underwater room to open the central structure and collect the quad damage.
  
• In the same underwater room as #1, swim up the hole at the left of the button to find a secret room containing a green armor, some shells and health.
  
• After picking up the silver key, you ride a small elevator and then drop down to the corridor located prior to the pendulums. A panel in the wall will open up at the other side of the corridor revealing a yellow armor, shells, some health... and a death knight.
  
• After opening the silver door and passing the lava pool you will come across a room with two buttons. Press the left one and the ceiling will start to go down. Place yourself under the hole to avoid being squished and then use the ceiling as it rise back up to reach a passageway at the to of the room leading to a red armor and rockets.

There are 6 secrets here.

• At the start of the level after passing the big arch, you end up on a terrace from where you can see the gold key in the distance. Jump over the small fence and drop down below on the platform with the two big wooden beams to find a quad damage, multi-rockets and lava nails.
  
• In the room containing the Super shotgun, there is a round stained glass window behind it. shoot it to open a panel beside the entrance and reveal a green armor.
  
• In the big room before the underwater zone, shoot the button above the entry gate to open two alcove from either side of the lift containing a biosuit, lava nails, rockets and shells. The two count as a secret each.
  
• In the underwater zone right after #3 and #4, shoot the blinking light on the ceiling to open a concealment nearby containing a 100 health.
  
• After entering the gold door, part of the wall on the left is actually not solid and hiding a red armor, rockets and multi-rockets.

There are 4 secrets here.

• In the large cave where you have to activate 3 buttons to open the exit, there is a shootable button located on one of the two walls surrounding the floor button, not far from the nailgun. It moves a big rock at the end of the tunnel to the right of the exit gate, revealing a ring of shadows and shells.
  
• In the circular room with wooden walkways, dive in the water and look behind the wall oposite of the exit to find a red armor, multi rockets and lava nails.
  
• After the ride on the moving cage, look under the bridge planks for a shootable button wich open a wall nearby containing 100 health, cells and shells.
  
• In the last room of the level you have to make your way to the exit on a collapsing wooden walkway over lava. At the begining of the walkway there is a small metal platform on the left that will lift you up to a previously inaccessible other walkway with a red armor and some health. The lift only became active when you are right bellow the exit tube, so you have to walk all the way up to the tube and then jump back down at the begining.

There are 7 secrets here.

• In the big room with the grenade launcher, on the left, there is an innaccessible cage over the lava with an ogre and a 100 health. Shoot the twitching light in the hallway at the other end of the room to open a teleporter leading to the platform.
  
• In the next room where you have to push the button at the end of the walkway, just before going up the ramp leading to the lift, a panel will open right behind you releasing knights. Their hideout contains a power shield, a green armor and some cells.
  
• In the room with the silver door and super shotgun, look behind the stairs for a shootable button. It opens a trapdoor beside the water basin containing a pentagram of protection, cells and nails.
  
• In the silver key room, shoot the wall in front of the key's cage to reveal a quad damage.
  
• After picking up the silver key and taking the lift, there are some stairs leading down to a water pool. Look behind the stairs to find a biosuit.
  
• After opening the silver gate, you end up in a room with the gold key. From the platform you landed on, jump on the right circular platform, reach the small platform above the button and then climb on the metal pipe to collect a 100 health.
  
• After opening the gold door and activating the lightning machine, you reach a library displaying some books with an eerie looking face on them. Press the one closer to the lift to reveal a quad damage.

There are 5 secrets + 1 "extra" here.

• After the first underwater cave you visit, you end up in a room with a teleporter pad in the middle and a row of torches on the ceiling. Hit the beam sticking out of the row at the other end of the room and a teleporter will appear, leading you to the pentagram of protection next to the silver door.
  
• At the end of the cemetery, you reach a space overlooked by a big red pentagram structure. Stand on the only tomb that remained close and climb the small ledge on the supporting beam, then jump on the ledges on the wall to finally reach the top of the structure and collect a red armor and some rockets.
  
• In the room with the gold door and unholy altar, shoot the demon face in the ceiling decoration and a step will come out of the lava for you to pick up the thunderbolt.
  
• In the big hall behind the silver door, in the dead end corridor, hit the torch with your head and a teleporter will appear leading you to the upper walkway in the hall where there is a quad damage and some ammunitions pickups. You can also use the anti-grav belt found in the other corridor as an alternative way.
  
• After picking up the gold key, go back to the small unholy altar you passed by previously. It will now be open. Swim down the cave to collect a power shield.
  
• There are secret chambers located behind each one of the four stained glass windows representing the Four Horsemen of the Apocalypse : Pestilence contains a yellow armor. Death and War will grant you a 100 health. Behind Famine a teleporter will apear on the right wall leading you directly on top of the unholy altar of secret #3.

There are 4 secrets here.

• At the begining of the level, one of the buttons you have to push is protected by two rows of lava nail shooters. The last shooter of the left row do not seems to shoot nails and is actually a button you can push to reveal a secret room under the ramp containing a 100 health.
  
• After completing the buttons sequence you drop down a shaft to a room with a center pillar with four stained glass windows representing Death. Look for the one with the inverted texture and shoot it to pick up a power shield.
  
• In the catacombs, after crossing the water ditch you end up in a tall crypt with lots of vaults covering the walls. Some of them have steps sticking out that climb all the way up to a small alcove containing a quad damage.
  
• Jump in the slime under the drawbridge, go right and find the small chamber containing a power shield.

There are 3 secrets here.

• Above the gold door, there is a small corridor you can jump from across the building oposite. Shoot the left wall at the end of it to collect a red armor, some lava nails and multi rockets.
  
• Before entering the room containing the gold key, you have to go through an underwater tunnel. Shoot the wall located on the right prior to the cylindrical section of the tunnel. It contains a 100 health.
  
• After opening the gold door you find yourself at the entrance of the temple where the exit is located. Jump on top of the "quake" lantern in the middle of the stairway to be teleported on the ledges above, a 100 health pickup and some cells are waiting for you. The ledges can also be reached by standing next to the immovable statues, facing the temple and rocket jumping.

There are 2 secrets here.

• From the start, cross the bridge, turn right and dive into the hole on the right, so you don't get crushed. After pushing the button, drop down underneath the block moving up and down, and into the side chamber. Timing is essential here. Or you'll have a sortof flat feeling.
  
• Shoot the four numbers, one on each air duct. That activates a lift to a 'secret' goodie. When you reach the top floor, go behind the exit pod, there is the lift.

There is 1 secret here.

• After jumping across some moving tiles, right before a hump in the floor, turn right into a corridor full of nail shooters. There is a walk-through wall, right below the hump. Shoot the switch inside, some steps appears, leading up to the big medikit next to the moving tiles.

There are 5 secrets here.

• Not a secret: there is a shootable button at the entrance. It opens a pit right underneath. Goodie inside.
  
• When you see the gravity belt, look to the ceiling right. There is a red button. Shooting it opens an room.
  
• No wonder that Pharaoh never found the Americas. Building a boat on first floor inside a stone block... Get on the port railing astern, you can see some steps. Get topside, and pick up a goodie.
  
• Upstairs, after the mazes, there is a cobra statue. Facing it is a obelisk with a red light. Shoot the light. The cobra moves, jump into the hole in the floor before it shuts again.
  
• There is another cobra/obelisk in the other end of the corridor. Same procedure...
  
• At the exit, shoot the left wall. Teleporter behind it, leading to the last secret.

There are 4 secrets here.

• Right at the start, when you go down the elevator, there are two bookcases. Headbutt the wooden thingy between them. One of the bookshelves slide away.
  
• Right next to the silver-key door is this strange thing. There must have been a interior designer around. Jump over the spikes and hit the red pole. A secret area opens in the next room, the one with the auto-shooter.
  
• When you take the silver key, you look down on a yeechy-looking altar. Behind it is three candles, the middle one is dark. Shoot it. The front of the altar opens, revealing a button. Shoot it, one of the stained glass windows opens. Jump down and get goodie.
  
• You reach a bright stained glass window and a button at the end of a hallway. Shoot the window. This triggers a lift, that takes you up to the last secret.

Easter Egg - not a secret:
You will find a switch sticking out of the bookshelf. Press it. The second switch is the candleholder right inside the door, next to the crates. Headbutt it, and when you can see straight again, a door with some goodies have appeared. Grab the goodie (you're gonna need it in the yard!), and go spread some mayhem.

There are 5 secrets here.

• Ride the moving tile over the lava. Nearly straight ahead, there is a misaligned texture. Some light shines from it. That's a walk-through wall.
  
• After jumping from stone to stone over lave (and with some jerk shooting at you), you go past a sawblade in the ceiling. Right after the blade, shoot the wall with the skewed texture. Secret.
  
• When in front the giant... frog? Lizard? Never mind. Shoot the bastard in the mouth to make it stop zapping you. Get the bars to the left opened, and shoot the wall behind them. Yeah, right. Secret again.
  
• Further down the road, you'll pass under some long wooden beams. There is two skull pillars with a skull...something between them. Jump on top of the...something, and it opens a secret door. Inside is a teleporter which takes you up on the wooden beam.
  
• After you got the blue key and move out of the basement, you go over another lava-flooded area with nails shooting up. You reach a 'V' corner with two faces across from a snake head. Jump up and headbutt the snake head, a secret door opens.

The final 5 secrets in this mission pack are found in this level.

• Jump in the moat. There's a underwater cave with some stuff.
  
• There is some green goo, and a high-voltage discharge into it. High up on the insulator is a switch. Shoot it, it opens a small secret door.
  
• This is the squasher. You need to start it. If you get caught in front of it, shoot the button beyond the axes. Get behind the squasher (There is sufficient room behind it to survive), press the key on the back wall. It will open a secret room between the two blue-key doors.
  
• Darn cheap wooden floor. To get away from the charming, but rather waterlogged zombies, you need to run to the end of the two corridors marked with red arrows, and throw the switches. That opens the bars. There are two candles in each corridor. Shoot all four of them, a secret door opens opposite the exit.
  
• After you finish stumbling around in the swamp, you take a lift up to a nice, dry room. On the upper left-hand side, there is a red switch. Get up to the wooden thing above you. Secret. Mr. Teddy Bear in the next room may cause some trouble in the process.