There is a hidden yellow skull hidden in every regular stage of the campaign. A total of 180 skulls can be found. In order for a skull to be considered collected not only you have to exit the level with it but you also have to make it under the maximum allowed time limit for the level. Usually this isn’t difficult as the skulls are almost exclusively located just a little off of your main path (or directly on it). The exception is Forge-29 Hardcore that has a significant detour which can make it challenging – you have to be able to consistently get the best medal before you attempt it. I made a request to increase the time limit for this level because of this, maybe they’ll ease it down a bit.
Once a skull is collected it will turn green for all subsequent playthroughs. Note that the same skulls can also be found in the BFL levels (and they will be yellow as if never picked up even if collected in the regular version of the map) but those don’t need to be collected and they aren’t tracked in the game menus anyway.
Can’t miss it, near the exit.
Hidden inside the penultimate vase on the right.
Crypt-1 Very Hard
Above the spawn, behind you.
You can line up the shot to shoot both the skull and the button with a pistol bullet. Or just grab the skull by holding slide before starting with a jump for a boost.
Up in the ceiling in the middle of the room.
Unmissable, on the left of the exit.
In the spawn room right next to you you, behind and above.
Crypt-2 Very Hard
Stashed above the door opposite of the exit.
Near the exit at the ceiling.
Soon after you start there’s a yellow button on the left. It unlocks a crevice on the right just up ahead.
Upon entering the final large room look behind you.
Upon entering the final room with the cracked ground look right and down.
Crypt-3 Very Hard
Once you leave the starting room immediately look on your left.
From the spawn go to the extra room on your left. The skull is behind the altar. Time is a little tight in this one so use the yellow button to get out through a different door.
Directly above the spawn, look up.
Above the door just past the spawn room.
Hanging above the door opposite of the exit.
Crypt-4 Very Hard
Just under the slidepad in the final room.
In the last room, just above the slidepad.
Last room, on the right by the entrance to it.
Keep going forward from the spawn.
After sliding through a tunnel at the start look to your left.
Crypt-5 Very Hard
On the left of the exit there is a locked gate. Right near it there’s a hidden button. Press it and turn around to find the skull in a now open crevice.
It’s wide open near the exit, can’t miss it.
In the ceiling of the first room, above the small stairs within.
The skull is on a candelabra in the largest room. You can see it from the spawn but it may be easier to shoot it at the end (after circling back to the upper floor of the same area).
From the spawn go right, then right again into a small room. The skull is hidden deep in the ceiling.
Crypt-6 Very Hard
Proceed as usual until you reach the long tunnel that connects to the upper floor of the main large room. The skull is well-hidden on the left side of this tunnel.
Hidden within a torch in the largest room, upper floor. It is somewhat above the lever that you pull to enable the slidepads.
Upon stepping off the spawn area go left (upper floor). The skull is behind the head of the statue.
Mid-level you get attacked by three “crabs”. The skull is in the next room.
It’s in one of the urns near the exit, left side.
Crypt-7 Very Hard
Towards the end of the level. It’s near the two enemies standing by a barrel.
Pull the lever, go through the door, and slide through a small tunnel. The skull is directly above you in the next room.
It’s in the ceiling in front of the starting gate.
Hidden in a large urn in the penultimate room where you do the U-turn towards the exit.
Mid-level, in a grave niche (in the room before the one with the stairs).
Crypt-8 Very Hard
On the right side of the exit door there are several explosive barrels. Among them is a large urn that has a skull in it. Blowing up all barrels helps find the skull.
On the pedestal with the first lever there’s a button hidden on the side of it. Shoot it to reveal the skull nearby.
In the room where you kill the second enemy there is a lever hidden above. Use either the extruding platform on the left or a crouch-jump from the coffins on the right to reach it. The skull is behind the lever.
Upon descending two flights of stairs visit the optional room on the right before sliding into the small tunnel.
After sliding through a tunnel go to the optional room on your left (same one as in Normal mode) to find a slightly better-hidden skull inside.
Crypt-9 Very Hard
Shoot a total of four buttons to escape the starting room. You’ll see a set of stairs. Look up and right to find the skull above.
From the start, immediately after the first flight of stairs there’s a button that reveals a skull.
After the long sliding section immediately look up.
Shortly before picking up the carabine check the extra room on your right.
It’s under the stairs in the large room.
Crypt-10 Very Hard
Directly behind the spawn there’s a tiny room with the skull inside.
Tucked in the wall on the right of where the two blue barrels are.
In the room directly behind the two blue barrels.
Right below the exit door.
After shooting the first button and getting out to the “hub” room, immediately look back and up.
Crypt-11 Very Hard
The room with the first button (next to the carabine) also contains the skull.
After you take the second slidepad, defeat a trio of enemies, and slide through a tunnel behind them look up to find the skull.
It’s hidden near the first button further down the room. Maybe don’t jump down towards the skull though because you’d get stuck.
Behind the altar in front of you.
It’s in the ceiling, just look up.
Crypt-12 Very Hard
Hidden in a torch (the one on the right of the permanently locked door).
This one is very well-hidden. From the spawn (assuming you’re facing the altar) go to the blue gate on your left and look for the skull in the ceiling.
On the right side of the altar there’s a yellow button on the wall. Shoot it to reveal the skull on the other side across the altar.
In the room with the button, above the entrance.
In the room with two buttons and the assault rifle, above the entrance.
Ruins-13 Very Hard
It’s in the ceiling just before the exit.
In the room with the second lever. Hidden above the entrance.
It’s right in the explosive barrel, you’ll likely get it without knowing. The trick is probably to make you search the level for the skull that you already have on you.
On the left of the exit, under some wooden planks.
Shortly before the exit there’s an extra area somewhat hidden between the rocks. Deeper into this area you’ll find the skull.
Ruins-14 Very Hard
Activate the lever and jump down a flight of stairs. Then immediately after you slide through a small rocky tunnel you’ll find the skull hanging above and behind you.
Upon entering the final large room instead of going left go right to find the skull behind the table.
Look behind you just before exiting the level. A bit to the right.
Just before exiting the level look back where you came from.
Just before exiting the level look back where you came from (again).
Ruins-15 Very Hard
From the spawn drop to to the right (instead of left) and look for a hidden button on the ground. It unlocks the nearby door with the skull inside.
Unmissable. It’s on the right of the exit.
As you take the bouncepad to fly towards the poison-enabling lever, there will be a cave on your right. You’ll find the skull inside. You can grab it after using the lever but flying straight to it from the bouncepad is more convenient.
After breaking the door and sliding through a small tunnel behind it, look to your right.
In the last room, opposite to the exit.
Ruins-16 Very Hard
At the end after you stomp a trio of enemies there’s a button that activates the final slidepad. The skull is located deeper into that same room, just past the button.
Pull the lever and take the bouncepad to land onto the bridge. Instead of going left however, go right and walk into the room. There’s a wooden barrel in the corner with the skull in it.
Once you activate and take the bounce pad that sends you flying across the whole room, carefully look out for the skull hidden on the left side in a crevice. You need to hit it mid-air.
Hidden in a wooden barrel. After running past the still-locked red exit look towards a corner to your right (before entering the lever room).
Upon picking up the sniper rifle, immediately look up to spot the skull.
Ruins-17 Very Hard
It’s on the ground near the altar (at the head). You can notice it from the spawn by taking a step to the left and just looking down.
Immediately at spawn turn to your left to see a wooden barrel by the gate. The skull is inside.
This one’s impossible to miss. It’s in plain sight just above the first lever you pull.
On the upper floor there’s a small room with two distinct, bright green fires on a table (close where assault rifle is). The skull is behind that table.
On the chandelier in the main room.
Ruins-18 Very Hard
From the spawn go forward then turn left to face four enemies. Next to them is a button that opens a nearby door. The skull is inside. No red skulls in there though.
Directly from the spawn move forward and get behind a tomb in the wall. Press the hidden button there to reveal the skull nearby in the corridor.
It’s on the right of the exit, hidden by a yellow symbol in the corner.
Upon picking up the assault rifle immediately look down before jumping towards the exit.
From the start you take two slidepads and go through a small tunnel towards a spike trap. Look up after taking the tunnel to find the skull.
Ruins-19 Very Hard
After shooting the three buttons instead of jumping down towards the next enemies and stomping them jump to a wooden platform on the left of them instead. You’ll see the skull from there.
Upon shooting the three buttons and jumping onto the next platform below, look behind you to notice another button that opens up the room with the skull inside.
Upon reaching the first green grappling hook block, look up and a bit to the right to spot the skull.
Behind the second lever that opens the exit door.
At the end of the level there is a bouncepad. You’ll find the skull hidden behind some vases on a “middle floor” between the bouncepad and the exit platform.
Ruins-20 Very Hard
After taking the first slidepad, in the room with the second lever there’s a hidden button by the entrance. Press it to reveal the skull in the same room.
Mid-level you get the sniper rifle. There’s a flight of stairs just before it. The skull is hidden at the top above those stairs.
It’s just under the first slidepad.
In the room with a green acid pool below, two explosive spiders come out on a platform on the right side. The skull is in the room behind them.
The skull is at the beginning of the level. Jump down and look around the small ramp that leads to the lever.
Ruins-21 Very Hard
Take the slidepad and pull the lever. A reddish, pink-fireball enemy spawns on the next ledge. Walk into the room behind that ledge to find a skull hidden in some wooden barrels.
After using the first stomp and taking a slidepad look behind you to see the skull above. Try not to fall down though.
After stomping a trio of enemies below you and going through a short tunnel immediately look to your left before moving further ahead.
Towards the end of the level. It’s inside the room in the middle of the way back.
Early into the level jump onto the wooden platform with the very first enemy and look down a bit.
Ruins-22 Very Hard
After shooting the first button and entering the doorway, look to your left.
Easy to find. There’s a suspicious well-lit tunnel that you encounter after the second slidepad. It’s a dead end with a skull in it.
In the very last room instead of going left to the exit go right and up to find the skull.
The skull is hanging just above the exit, can’t miss it.
Mid-level there’s a lever to pull. The skull is just behind that lever, concealed by a barrel.
Ruins-23 Very Hard
Later down the level, at the second lever there are two locked gates on both sides. Shoot through the left gate to activate a button that opens the right gate. The skull is in that room.
The skull is just under the starting platform. Look behind after the first stomp.
Just near the exit, in a small room on the right.
It’s in an urn in front of the statues with yellow swords (left side if looking at the statues).
Behind the shield of one of the green statues.
Ruins-24 Very Hard
There’s a lever hidden behind some barrels on the left of the statue with a green shield. Pull it to unlock the skull nearby.
The only surviving column has a button at the very top. Shoot this button to reveal the skull at the ground level behind it.
The gate guarded by the yellow statues has three buttons around it. Shoot to reveal the skull above.
Just after all the flame traps but before you drop down. In a tunnel on your left.
Under the first slidepad.
Forge-25 Very Hard
Near the end of the level, in the lava pit room. Look behind where you came from to see the skull hanging above.
Make your way to the third lever. There’s a lava pool with a short pillar behind it. The skull is hidden behind that pillar in the lava.
First you activate a lever, then a set of two buttons. Upon doing the latter and turning back to proceed the skull is visible on the right of the cave.
Can’t miss this one. Just at the start of the level after you kill one enemy the skull can be found at the lava (somewhat behind the second enemy).
Under the second platform you jump down to. Can’t miss it, you’ll see it above after pulling the lever and turning back.
Forge-26 Very Hard
It’s in the room with the second lever. Check back where you came from after pulling it.
Make your way through this level until you reach the second lever. Activate it and go back. You will now have to use the grappling hook to reach the exit door. Look to your right as you do it to spot the skull.
Activate the first lever and turn around. A lone enemy spawns on the intermediary platform on your left. Jump onto it and look down its edge on the left side to find the skull.
The skull is just above and behind the second lever that you pull.
It’s just to the right of the sniper rifle, tough to miss it. You can shoot it from the previous platform before taking the slidepad to avoid messing up the weapon switching.
Forge-27 Very Hard
Unmissable, it’s directly on your path. At the flying enemy that you fly into.
As you’re hopping through lava towards the final third lever you can spot the skull up ahead, close to the lava.
Almost at the very end of the level you take a slidepad that sends you into bouncepad (past a flying enemy). After that instead of going straight to the exit take a sharp right to notice a rocky ledge with a distinct blue light in it. The skull is inside the cavern there.
Look up after using the grappling hook two times.
The skull is in a small crevice on the left of the second grappling hook block. Look behind you as you descend into the final long room.
Forge-28 Very Hard
Close to the exit there is a hidden crevice on the right side, down at the lava. You can stand on the edges below without taking damage.
Just near the exit there’s a small extra room on the left side with the skull at its entrance.
The two very first grappling hook blocks are located one above another. Hook up to the top and look left. There’s a small extra room with the skull inside it.
Can’t miss this one – after taking the first bouncepad look up and left for the skull above.
Mid-level you shoot two buttons and then drop all the way down to pull a lever. The skull is hanging directly above that lever.
Forge-29 Very Hard
In the same room where you pull the first lever there is a small pipe sticking out of a wall. The skull is hidden inside.
Towards the end of the level you encounter a trio of backflip enemies. Deal with them and jump down to find a button under a ledge. It opens the door just above the button. Use the nearby bouncepad to get into that room and go all the way in to find the skull. This is a long detour so the time limit can be challenging here.
Upon picking up the rocket launcher and blasting away the two groups of enemies in the room, look for a hidden button in the ceiling of the same room. It reveals the skull nearby.
From the spawn turn around and go back. See the platform below with a slidepad on it? You need to get to another small platform closest to that one. The skull pickup is hidden in a crevice in the wall there.
In the largest room, on one of the highest platforms there is a large pink-fireball hulk guarding a platform with lots of skulls alone. The skull pickup is in a crevice behind him.
Forge-30 Very Hard
Let’s go with Patrick’s route – right slidepad at the start, then right again. Eventually (~62 seconds left, ~170/300 skulls) you’ll reach a dark area where a few bouncepad bump you back and forth very fast (faster than in any other part of the map). The skull pickup is on the next platform, at the next bouncepad. It’s easy to spot. You have no bullets but you can use the stomp ability to forcibly stop at this platform (surprisingly this might take a few tries as the bouncepad often triggers again upon landing).
Let’s follow Patrick here. He collects 5 skulls first, then grapples upwards to collect the 6th on a tall pillar above. That’s where we have to go. From that skull carefully jump onto the adjacent metal tube and walk around it towards the dark corner of the map. You’ll see the collectible soon enough just below. You can also just mindlessly dash into the dark corner but getting out might be tricky.
Count with me! Upon getting to the 14th grappling hook block jump down below it and turn around. You’ll see an obvious yellow button located on one of the pillars. Shoot it and proceed through the level – you’ll see the collectible dead ahead of you. Friendly reminder that you can shoot that skull unlike all the others ones to avoid wasting time trying to pick it up.
Near the start. Look on your sharp left immediately after taking the slidepad.
Mid-level you jump down to stomp the enemies, then turn around 180 to slide into the next area. Look up after stomping to find the skull above you.
Forge-31 Very Hard
After you get a rocket launcher and unlock large circular gates, the skull can be seen directly above past the aforementioned gates.
Navigate the level until you reach the part where you do a U-turn into a lava river that you hover across using two grappling hook blocks. Before hooking up to the first block look all the way up to find the skull hanging near the ceiling.
Mid-level you pull the lever and soon after that you proceed to climb up by using the grappling hook blocks following an upwards spiral of sorts. In this area just after the first blue block check the walls on the right to notice a small cave with a blue light in it. The skull is inside.
It’s directly under the starting platform.
As you enter the last room and approach the horizontally moving platform look back where you came form to spot the skull.
Forge-32 Very Hard
Mid-level you get a rocket launcher and take out the next two groups of enemies with it. There are two grappling hook blocks leading up to a short tunnel above. Check on the right side of this tunnel entrance to find the skull.
After using the first grappling hook block, instead of proceeding to the left check the right side first. The skull is there. Rockets won’t collide with it though. Switch to pistol for which you are generously given one bullet.
There are two sldepads in the later section of the level. The skull is near the second slidepad, hidden behind a wall on the right.
The skull is in the air directly on your path, can’t miss it. Switch to a different weapon because for some reason rockets don’t collide with skulls (not that you’d want to waste one anyway).
In the room where you spiral down counter-clockwise. The skull is at the column in the middle of that room.
Forge-33 Very Hard
In the last area when you jump onto the curved metal/rock structures look behind you for the skull.
Mid-level you take out a flying enemy, dive all the way down, and then use a slidepad. This next room has the skull in it behind the pillar. It’s just before the next part where you stomp from one platform to another.
The skull is hidden at the massive circular bell-like structure. You use grappling hook blocks to fly past it and climb onto an edge above – look behind you once you make it to the top.
It’s hanging above, visible immediately after performing two rocket jumps. Can’t miss it.
Unmissable. It’s right out in the open in the first room, on the right.
Forge-34 Very Hard
At the beginning you shoot two buttons to open the gate and fly into it with the grappling hook. Look behind you once you go past the now unlocked gate.
Near the end of the level there’s a room with two buttons that you need to shoot. When exiting that room check your immediate left for the skull.
After pulling the second lever don’t proceed forward just yet. Instead explore the blue platforms on the left of where you need to go. You’ll find the skull hidden there.
Near the end of the level, close to the lava. It’s barely visible if you come out of the room with two buttons and walk as close to the edge (forward+right) as possible.
Directly from spawn walk forward to the edge and look down to see the skull. You can choose to shoot it later though as you circle back through the bottom floor of this area later down the level.
Forge-35 Very Hard
Climb to the top to press the three buttons then stomp your way down. Before taking the slidepad in the next area look behind you to spot the skull.
It’s hidden in the area where you repeatedly hook upwards in a spiral around a tall platform. If you go all the way to the top, pull the lever, and walk back to the edge, the skull would be in the far right corner.
Just after picking up the second rocket launcher drop down into the lava and look to your left. There’s a small secret room with a lever inside which reveals the skull. You still need to complete the level though so perhaps shoot the elevated enemies with the rockets before dropping down.
Start the run and slowly turn right until you see pink fireballs coming at you. Below the meathulk that fires them there’s a barely visible wooden barrel (lower platform, to the right of the center). The skull is inside it.
It’s just under the central circular area. Jump to the left from the spawn to see it.
Forge-36 Very Hard
The skull is visible from the start (near the ceiling on the right). Problem is, shots go straight through it – you have to actually grab it. Reach one of the higher platforms (dark ones) under it, then wallclimb with the rocket launcher. Alternatively use either of the grappling hook blocks to activate another one hidden all the way up in the ceiling in the middle – this is likely the intended way. Wallclimbing might be easier.
Turn around 180 degrees and grapple your way onto the platform. Take the slidepad to your right. The lonely barrel on this platform conceals the skull underneath.
From the spawn, jump to the left onto the platform below you. Move forward to take the slidepad (i.e. go clockwise). You will now arrive at another such platform with a lone barrel on it. The skull is hidden on this platform by the wall, behind some rubble.