If you remember that big long ramp you were sliding down at the very end of the original game then you will recognize your starting location at the beginning of this level as you finish the slide. Proceed forward and jump over the pit then head to the boiling lava and jump over it. Grab onto the ledge and pull up and head to the right to claim SECRET #1: 4 boxes of shotgun shells. That was easy!
Jump back over the lava and head to the pit and drop down. Head up the ramp to the right. A boulder will start to roll towards you but it will fall in the pit before it hits you. A running jump will send you over the long pit with the boulder in it and you can enter the next room. Ignore the creatures to the left - they can't hurt you from behind the wall. Grab the medkit hidden in the alcove behind the small lava pit, then head into the next room and kill the demon which attacks.
Lots of shotgun ammo is to your left and back behind you. Now head to the edge which overlooks the huge area below. The large tower structure has a ledge on the right side. Drop down onto the ledge then pull up into the top of the tower. Jump across the middle of the tower so you land on the ledge about halfway down. This triggers a pair of demons which you can easily shoot as they circle the tower. You can climb back up one level for a better shot if necessary.
A medkit and shells wait for you at the bottom of the tower, so do a safety drop from the middle ledge to avoid damage from the lengthy drop and get the items. Head to the opening that overlooks the large sloping surface with the four square holes. Jump out towards the left corner of the building. A boulder will come rolling down the hill so get out of its way fast and jump up to the path above! Grab the shotgun shells. Follow the path until you can jump up to the ledge that circles this cavern. Make sure to avoid another boulder trap as you land. Now make an angled jump towards the square pillar and get some more shells then take out the attacking demon. More shells and a medkit wait for you below and another pair of demons are waiting to move in. When you have the items and the demons are dead you can now enter the sloped structure. Do a safety drop into the hole then jump down to the lower ledge then jump across the two pits and collect a pair of medkits. Now go to the shaft which is to the right of the rock ledge
and drop off and hang from the edge. Shimmy to the right before dropping to the pyramid below. Kill all three demons and get all the items which consist of six boxes of shells and a medkit. Two of the demons will hatch from the green pulsating pods on the wall while the final demon waits for you to grab the shells by the pyramid.
Head through the large door and run down the passage with the dart trap until you reach the stairs. Go down and kill another demon and continue into the next demon-infested hall. Three demons are frozen along the walls and will not wake up unless you take the items they guard. Unless you are going for a perfect item or kill score leave these guys alone. Go to the bottom of the stairs and flip the switch and take the shells before heading up the stairs. Flip the switch at the top to wake up one of the demons. Uzi's or magnums work nicely as he comes up the stairs towards you. If he is still alive after than then drop off the landing and finish him
as he is coming back down the stairs. Repeat this process by flipping the remaining switches to activate the other two demons. When all three are dead you can take the items they were guarding; 2 magnum clips and 6 boxes of shells.
Head back up the stairs and get the medkit (if you haven't already) and flip the two switches by the membrane to open doors in other parts of this level. A pair of demons guards a medkit, 2 magnum clips, 2 Uzi clips, and 4 boxes of shells in a small room to the right. Kill the demons and load your backpack before heading back up to the room with the pyramid. Go to the new door and kill the demon along the way. Head up the ramp then climb up onto the ledge then turn and jump to grab and pull up to another higher ledge. Jump to the next ledge and over a lava pit then go up to the left and kill the demon at the top of the ramp. Continue on until you reach the next room where you can get a medkit then jump some lava to get 2 boxes of shotgun shells.
Return to the bottom of the ramps and head up the right ramp and jump into the opening at the top. You land inside a room with lots of flying darts and a pair of demons. Kill them then take a swim to find a pair of switches which open an underwater door. Swim through the door and follow the passage to obtain SECRET #2: medkit, 4 boxes of shells and 2 magnum clips. Before you leave the area you will also find another medkit and 2 boxes of shells under the
water (not part of the secret). Once you are done swimming you can continue out of this room and into the next smaller area. There is a switch in this room but ignore it for now and instead, go to the room to the right of the switch where you will find a pool and another switch. Flip the switch to release 4 demons who will attack from the previous room. Kill them all then dive into the pool to retrieve a box of shells. Return to that first switch where you will find another passage you can follow to another switch. This switch opens a door nearby which leads to a pair of 
medkits. Now go back to that first switch and flip it to open a pair of doors which hides a pair of demons and a total of 8 boxes of shotgun shells. There should be one last switch you haven't flipped back in the pool room. Flip it now to open a door back to the main cavern from the beginning of this level. A switch in the bottom of the pool will open the door at the top of the steps. 

Go through the door and kill the demon then head left and kill another demon. Four magnum clips lie in the bottom of the pool so dive in and get those before flipping the left switch to open a tunnel which leads to some shotgun and Uzi ammo. Climb out of the water back by the left switch then climb up the two ledges on the left to find another switch which opens a door above. Jump the gap to your left and pull up to find some shotgun shells on a ledge next to you. Return to the ledge with the switch and climb up the opposite wall to retrieve a medkit. Jump over to the ledge with the door and enter to find a room full of spikes and other useful inventory items. A jump to your left should put you on a large ledge. Go up the slope and jump up to the ledge to retrieve the medkit before dropping back down and doing a running jump to the ledge with the magnum ammo. Drop and hang off the edge of this ledge then drop and grab onto the crack just below the edge. Now shimmy to the ledge on the right and drop down to get more magnum ammo. Pull up onto the next ledge and drop down on the other side then drop to the clear spot in the
room below and walk through the spikes to obtain another medkit. Flip the switch before walking back to the clear spot in the spikes. Dive in and follow the underwater tunnel, making sure to get the magnum clips and shells located in the tunnel. Flip the left switch in the next room then dive down and head through the door on the right which opens by itself. Return to the spikes and exit the water and walk through the spikes to your right. Climb up to the first ledge then turn left and climb to the next then jump into the small alcove then turn and jump back to the ledge. Jump and pull up to the ledge on the left and continue until you see a large stone bridge. Jump to the bridge then do a running jump over the large chasm and
kill the demon. Shotgun and magnum ammo are your rewards for this encounter, but they also trigger a second demon as you are picking them up so kill him too. 

Jump back to the bridge and head for the alcove to the right. Pull up to the ledge and head for the lava tube. Jump past the two lava jets and get the shotgun shells. Now head back down the lava tube and pull up onto the concealed ledge on
the right and enter the tunnel. Follow it to the ledge with a spectacular view of the main cavern. Jump to the next ledge and kill the pair of flying demons who are coming in for the kill. Grab the shells and the medkit before doing a flying leap to the small ledge with some magnum clips. Jump to the large platform and make a note of the arrow carved into the stone. That is your guide to jumping into that hole in the top corner of the sloped structure. A running jump should send you sailing into the hole for a triple-scream fall before you splash into the water below. A Centaurian patrols this area so climb out of the water and eliminate him before heading through the door and down the hall. Flipping the switch at the end of the hall will end this level.