At the beginning, the game introduces you to fighting. Push FIRE to kill the two former humans and the former sargent. Next, a green light lights up the door and the game tells you to press USE on it. Grab the red key while you're here.
Once the door opens, sneak up on the former sargent to your left. Once you kill him, run around the corner and kill the single former human in front of the door. Open the door and kill the former human inside. You will see a single former human standing on platform. Jump onto it and punch him off. Next, run along the platform in front of you, jumping on the spring and jumping off the two springs at the end of it to bounce back to a platform at the back of the room. You'll see three former sargents. Push and hold ALT FIRE to use your "chainsaw attack", which can dig right through these three. Next, get a running start and jump off the edge of the slope ahead. You'll pick up the blue key. Optionally run-jump down to the platform jutting out of the building and grab Oswald's Helmet. You can now exit this room, having the required key.
Next, turn left and keep running down the hallway until you see many former humans. Enter the room to your left. There are 6 former humans in here. Kill them, then run-jump across the rocky ground which reveals to be lava if it collapses. Open the door and use your "chainsaw attack" (ALT FIRE) on the former humans, as you are forcibly moved across the room. You should pick up a pistol and the yellow key. You can now exit this room, having the required gun and key.
Next, kill all the former humans in the hallway. Be careful of ammo, however. Kill most of them in the middle, then use your fists. Hide behind walls to recover your health if it gets too low. Once you kill these former humans, proceed into the tiny door against where the wall meets a corner. If you're going the way you were going, you'll see it on your left. Go around the corner and equip your pistol (2). Shoot at the two former humans and former sargent. Once these are killed, continue to move around the room. You'll run into two demons. Stand back and shoot at them from a distance, as they explode when killed, causing severe damage if you stand near them. Once you kill them, continue walking around the room until you reach a dead end. The middle of the room will open, revealing a dark sky and four pink blobs. Kill the blobs using your rapid-fire (ALT FIRE), then pick up the shotgun. Don't forget the ammo, though! You'll need it.
Now that you have all three keys, exit back to the hallway and go the opposite way back. When you reach the big door, open it and proceed to the giant cup at the center of this open area. Step on the crack and the ground will cave in. Kill the demon in the tunnel then proceed in.
You will be in a very dark area. Save here, then kill the imps in front of you and the demon to your left. back into the tunnel to your right when they trap you, as the gargoyles will trap you against walls. Use the rocket launcher you earn in this room to shoot them while they're far away from you, but be careful as they can fly at you at random. Once they're dead, go across the room where the demon came out of to find four more demons. Kill them, then enter to find ammo for the rocket launcher and shotgun. Exit that area and navigate the various tunnels. One tunnel close by has mice in it. Mice charge at you fast and kill you in a matter of seconds, while you can kill them in one shot, so switch to your pistol and save often!
Once you go down the mice's path, you should see a blue skull. Pick it up and back up out of that tunnel into the area where you found the rocket launcher. Then, go down one of the paths with the automap up to see when a line finally appears down here, as none appear to help make you lost. Follow the lines to find a secret area that has a door. Open the door with the key you earned.
In the swastika-shaped room, go right to find bars. Push USE to lower them and get a Megasphere. Then, back out of the swastika, and find the side of the swastika that leads to an exit. This leads to the Rock Tunnel. Save here, then enter. Kill the demons running at you with the rocket launcher or pistol's rapid fire, then use the springs to reach a secret that contains Kai-lan's Dinosaur Helmet. Grab that, then open the "door" to lower the wall and reveal an opening with a Spyro basket. Break it and grab the ammo that comes out. There are many Spyro baskets around this cave and this whole hack too. Each give you random ammo, which is different from the ammo you've seen before this point. Then, run up the path killing every enemy in your way, watching out for the Angry Hammer Bro. on a high platform nearby. Also, watch out: Around Rock Tunnel, weak gargoyles spawn randomly. Be sure to get out of the rock tunnel fast, before they drain your ammo!
Once you found the end of the path, walk into it to find long piece of land that leads to a low area. The game tells you with a blue light to crouch. Push CROUCH to crouch. Unlike the others, this button is not assigned by default. Once you crouch under the low area, you'll run up a slope around corners to escape the cave. Then, run around the corner to finish this level.