If you intend to attempt Nightmare difficulty, the portal is in a hidden section of the difficulty selection level as usual:

Enter the section where the "Easy" slipgate is located. Once you're at the archway, turn right and follow the hallway. Go around a corner - there should be a stack of boxes on your left. Jump up and find a path through the crates, and eventually you will come upon a lift, inside the area protected by a force field. Ride the lift up and it will take you to the slipgate that selects Nightmare difficulty.

This level has 5 secrets:

• On the second floor of the main spiral staircase, turn left and shoot the Wall next to the staircase leading to the third floor to open an alcove with a Red Armor.
  
• In the corridor that blocks access to the level exit, look up and shoot the light that appears more yellow than the rest. A Computer Screen will open to reveal a Ring of Shadows.
  
• In the outdoor area, go under the bridge. Facing away from the door leading to the level's exit, look up and shoot a small Shootable Button. The Wall will open to reveal a Proximity Launcher, Shells, Nails, Rocket (Q1)|Rockets, and Cells.
  
• In the two-level Water reservoir, there is a staircase connecting the two levels. While on the staircase, look up to see a Shootable Button. Shoot it to gain access to a 100 Health.
  
• In the room with the exit Slipgate, go to the Crate to the right of it and jump to see a small Shootable Button on top of it. Shoot it to create a staircase next to the Crate, allowing you to climb up and reach a secret area with 100 Health inside.

This level has 3 secrets.

• There is large ramp near the level's start. The area can be reached through a small window further in the level. A Wall to the left of the ramp is slightly sunken inwards. Shoot it to reveal a Quad Damage.
  
• In the room with several stacks of Crates, face the archway, and then jump onto the stack to the right. Jump across to the stack on the opposite side of the archway to get a Yellow Armor.
  
• There is a Shootable Button above the doorway leading to the large room with the force field bridge. It opens the Door directly across from the Silver Keycard, yet for a very short time. Using your Proximity Launcher stick a proximity grenade directly on top of the button and run to the platform where the Silver Keycard was. After a few seconds the proximity grenade will explode and activate the Button, opening the Door and creating a Bridge for you to cross which leads to the Military Complex.

The first secret level contains 7 secrets.

• From the start, enter the Water and go straight until you reach a turn. Surface and look up to see a small Shootable Button. Shooting it will raise you to a small hidden compartment with a 100 Health inside.
  
• In the area where you have to press three Buttons, one of them is located in an outdoor area. Look inside the tower-like structure and shoot a Shootable Button on its ceiling to allow you to get into the tower, where a 100 Health can be found.
  
• There is a small outdoor are with an L-shaped bridge. While on the bridge, jump down to collect 3 Nails.
  
• A balcony, where one of the three Buttons you need to press is located, overlooks the level's starting area. Jump down to the ledge surrounding the area and make your way to a darker corner, where a Super Nailgun can be found.
  
• After going down the long dark descending ramp, turn right and you will be in a room with a stack of Crates to the right. One of the stacks has a red arrow on it. Using the other stacks, jump on top of it. Once you are there, another Crate will lower to reveal a Red Armor.
  
• When you enter the sewer room, jump down in the Water, and go back to where the room's entrance is. Underneath the walkway is a Quad Damage.
  
• After going through the Silver Door, proceed down the corridor until you reach a room that is split by a Water canal. Shoot a Wall that looks different from the others and enter a small chamber with a Yellow Armor inside.

This level has 3 secrets.

• After the tunnel with Rolling Boulder, go down the next tunnel. Jump onto the pipe to the left and follow it to a Wall that opens to reveal a small room with a Red Armor inside. Leave the room from the opposite side to find 2 Cell.
  
• In the room with the Radioactive Container, jump onto the rail sticking out of the ground, then get onto the nearby boulders. On the floor is a Button that opens a nearby Computer Panel with a Ring of Shadows behind it.
  
• In the area before the rotating tube with small boulders inside, examine the side passages for a Button. Press it and you can jump up the red bars to an overhead walkway with a 100 Health.

This level has 3 secrets.

• The room with a Silver Key: go up the ramp, when you reach the top you hear an opening door. Drop down to bottom to see a new opening.
  
• After the Silver Door, up the lift, you reach a room with some terminals. At the other end of the room, one row of terminals has a button at the side. Press, 2 terminals slide open.
  
• Facing the Gold Door, turn left and shoot the small blue light.

This level has 8 secrets.

• When you stay before first doors - turn left and look for a shootable button. After you shoot it - you can dive to the pool to get 100 Health and 40 Shells.
  
• In the room with Nailgun and the metal door - part of a wall under the wooden ramp is different. Shoot it to get Red Armor.
  
• Jump to the opened metal door, there is a button here. Press it and jump to the opened area to get Proximity Gun.
  
• There are 3 sequence buttons around the room with the blue key. The ladder with Horn of Conjuring have a shootable button exactly opposite the sequence button. Shoot it and get Quad Damage in the opened alcove.
  
• The ladder with Green Armor - there is a shootable button above. Shoot it and get 100 Health.
  
• There is Empathy Shield in the room behind the silver key door. To get it - blow wooden bars with a grenade/proximity grenade.
  
• When you reach the ramp above the metal door (before the external gallery) - shoot the square window to get Ring of Shadows.
  
• There is a shootable button above the level exit. Shoot it to lover the barrier in the Secret #3 and go to the secret exit to HIP2M6: The Gremlin's Domain.

This secret level contains 6 secrets.

• Down the stairs from the beginning of the level, there is an Ogre behind some metal bars. Shoot the wall to the right to open it and gain access to a staircase leading up to the Ogre and some yellow armor.
  
• the square room with the metal grate leading to water, located down the stairs to the left of the gold key door, turn around to face the stairs and look up. Shoot the button on the ceiling and the pillar in the middle of the room with the gold key door will lower, revealing a 100 Health pickup. Look up and there is a shootable button which the pillar in the middle of the room once blocked. Shoot it, and one of the stone faces will open and a staircase will slide into place, allowing you into the secret which will have the Thunderbolt.
  
• After collecting the wetsuit, open the door to the stairs leading into the water and look up. There is a shootable button. Shooting it will open the ceiling at the end of the underwater hallway before it turns to the right, revealing a small room with a Quad Damage inside.
  
• the room with the silver key (found right after getting #4), the diamond shaped piece of the floor the key rests on is a button. Pressing it opens the wall to the left, which leads to a secret elevator and the Mjolnir.
  
• Shoot the paned glass window facing the silver key door to reveal a teleporter that will take you to the room with the Death Knight under the large grate near the beginning of the map, containing Rockets and a Red Armor.
  
• In the room after going through the gold door key, there is a stone face which is glowing yellow. Shooting this wall will make it open, revealing a Ring of Shadows.

This level has 5 secrets.

• In the corner of the library room, there is a book sticking out of a bookshelf. Push it to make the wall revolve, revealing a secret room.
  
• At the large cathedral hall, look up and shoot the Button on the ceiling. When triggered, it will cause one of the pews to slide revealing a teleporter.
  
• the room that contains the button to open the tower, there is a distinctive door-like panel on one of the walls. Shoot it.
  
• In another bookshelf in one of the corridors, you'll find another book sticking out. Push it.
  
• In the room with the dropping floor segments above a pit of lava, there is a shootable plate located above the entranceway.

You can find 4 secrets here.

• In the first cavern after crossing the bridge and before jumping above the lava, if you look at the walls you will see some stones sticking out and forming stairs, go up to collect cell.
  
• In the fortress you will come across a corridor with a grate on the ceiling and a large door. The torch located at the end is a switch, jump under it to move it and reveal a hidden door with a teleporter leading to a secret chamber containing the Super Nailgun.
  
• After going down the spiral staircase, you reach a small room with the golden key door at your left, shoot the glowing cross in front of you and a 100 Health will fall from the ceiling.
  
• As you pass the first set of stairs after the room containing Mjolnir, some bricks will pop out of the wall on your right. climb them and follow the path to collect the Empathy Shield.

This level has 6 secrets.

• In the room with the coffins and tables, jump on the stepladder and then onto the top-left shelf (the one Cells are placed on) to reveal an alcove on the other side of the room with a Horn of Conjuring.
  
• In the same room as #1, walk onto the open coffin with a skull symbol inside to be teleported to a secret with 15 Health items, Shells, Nails, and the Mjolnir.
  
• Use the jump pad in the corridor to the right of the beginning of the map to reach the lever that gives you access to the Pentagram of Protection. Before you touch the lever, turn around and jump on the beam behind you and then on the ledge in front of you: the wall will open to reveal a secret with Rockets.
  
• After collecting the Pentagram of Protection, run to the scaffold with the Quad Damage and jump into the lava which is infested by Scrags, and swim back to reveal a secret with 100 Health. Revealing this secret will trigger a message reading "The Levelord Is Impressed!"
  
• In the tombstone workshop, jump on top of the tombstones piled into a corner to reveal a secret with an Empathy Shield.
  
• Right of the alcove with the gold key, look at the wall to the right to see a small door. Jump on the ledge and shimmy to it to reach a secret with Shells, Nails, a Laser Cannon and a platform that will take you to the Red Armor visible from the scaffold.

There are 6 secrets here.

• In the room before the silver key room, go up the stairs and jump on top of the crates, then on top of the ones next to the door: a door will open revealing a teleporter to the top of the room, where you can collect a Quad Damage.
  
• After opening the silver key door and reaching the path overlooking the starting point, jump on top of the gate to open a small alcove containing Cells and the Mjolnir.
  
• In the room with the door that opens via two shootable buttons, jump onto the button in the middle of it to open a door to one of the buttons, where a 100 Health is guarded by a pair of Ogres.
  
• In the room of the gold key door, shoot the demon face above the archway to open the cage next to it and collect the Nails. You can quickly jump on the bar as it rises so you can jump on the columns siding the archway and collect the Cells.
  
• On secret #4, instead of jumping to the Cells jump to the wooden wall on the other side, revealing a secret containing a Red Armor.
  
• Past the gold key door, right below the platform where you are there is a barely visible button. Press it to open a secret on the other side of the room containing a Yellow Armor.

There are 6 secrets + 1 "extra" in this level.

• Peek out from the trolley's starting point to the left to see a door. Walk along the thin ledge to reach it and open a secret alcove containing Rockets.
  
• As the trolley leaves, jump to the right on top of the roof to collect Nails, Rockets, and a 100 Health off the left side of the building.
  
• From the roof of #2, jump to the ledge along the cliff to collect more Rockets and trigger another secret. If you go further and keep climbing along the cliff, you will eventually find a very thin ledge: if you can walk along it, you can sequence-break the map and directly reach the top of the tower with the silver key, triggering a message that reads "You're not supposed to be here!"
  
• In the tower's middle floor (the one with the large grate), look up to see a button. Shoot it to open the large demon face below you, revealing two 25 Health and a Ring of Shadows.
  
• In the corridor next to the same floor as #4, approach the leftmost of the three demon faces to open a door to a secret with a Quad Damage.
  
• In the large stairway room, shoot the brick with a cup mark on it to make two more bricks pop out and a 100 Health appear on a nearby ledge. Jump across them to collect it.
  
• Behind the left torch near the gold key door there is a teleporter that will warp you to the top of the exit room where a Pentagram of Protection is located.

There are 5 secrets in this level.

• Right at the starting point there's a platform above you. Shoot the green light to the left to open the wall behind it, revealing an alcove with the Nailgun.
  
• In the following room, shoot the light facing the elevator to access the small room with the Grunts next to the corridor, collecting a Yellow Armor, Shells and Nails.
  
• After opening the barrier to the Silver Runekey room, walk through it and shoot the wall immediately to your left to access a secret alcove with the Thunderbolt.
  
• Shoot the wall behind the Silver Runekey to reveal a teleporter that will take you above the room's ceiling, where you can find two Rockets, two Cells and three 25 Health. Be careful not to fall into the lava pit when you drop back down.
  
• After collecting the Gold Runekey, look down to the left behind you and you'll see a red button. Shoot it to open the wall on the other side of the platform, revealing a 100 Health.

You can find 4 secrets here.

• In the room where you are ambushed by Gremlins, shoot the cross next to the stairs to open a secret on the other side containing Shells, Nails, Cells, and the Mjolnir.
  
• In the water section, jump into the water after collecting the Wetsuit and shoot the wall to the right of the grate to open it: go through it and emerge to collect a 100 Health (You can shoot a red rune in the corridor near the rapid-firing spike trap to open a trapdoor that takes you immediately to the Wetsuit's location).
  
• After opening the silver door, shoot the wall next to the gold key to reveal an alcove with a Yellow Armor.
  
• In the same room as #3, shoot the cross above the sealed tunnel to raise the bars, opening the way to the secret exit. This also opens the bars sealing the water pipe above the silver key court, which is an alternative path to this exit. (You have to first shoot the button above the waterfall as well.)

There are 6 secrets here.

• Right at the beginning, shoot the four demon heads on the walls to reveal a button that will make a Red Armor rise from the floor.
  
• After rising through the elevator shaft with the Scrags, shoot the demon head on the left to open an alcove in the right wall with a 100 Health.
  
• After opening the silver door, go up and stairs and jump on top of it to press a button that will open a door on the nearby wall, leading to a Pentagram Of Protection and the Hipnotic Software logo. A shootable button above will make the logo spin.
  
• In the same room as #3, shoot the wall behind the stairs to access a space with a teleporter to the top of the room, where you can collect a Quad Damage.
  
• On the bridge with the Vore, jump down to the right and swim through the opening in the wall to reach a secret area with Rockets and a Yellow Armor, and a teleporter that will take you back to the bridge.
  
• After emerging from the blood river section full of Rotfish, walk into the wall between the stairs and the large demon face overlooking the room to open it, revealing two Nails and a 100 Health.