Doom/E1M3: Toxin Refinery

It gets a little harder here. Grab the goodies and open the door. There are lots of enemies in this room; you'll be assailed immediately by an Imp and a shotgun marine, and around the corners there are lots more enemies including two new ones, a Demon and a Spectre (look them up on the enemy page if you don't know what they are). Your new chaingun will help here, otherwise continual pummeling with the shotgun will best them. Once they're all dead grab the goodies and go to the left. Open the door at the end of the path around the central slime pit.

In this next room, quickly blast the central barrel to take out the two marines on either side, and then let the several Imps and marines/shotgun marines come into your field of view and blow them away with your shotgun. There are a lot of them. Head to the window on the right by the stairs, and you can shoot some more Imps and marines from up here. Go around and kill any Imps that come up the stairs, then go down and clear the place out. There's some ammo and armour up the lift, but otherwise go left at the bottom and you'll find a switch. Throw it and head back up. You can open the wall section in the random alcove up here and shoot some enemies in the room beyond, but that's if you choose to go to the secret level or not (it's difficult, but fun). Anyway, go around back to the door and you'll find a wall section has opened, with a long staircase beyond it. At the top, snipe the shotgun zombies and Imps with your weapons, and head up. Clear the place out (there are some more Imps and shotgun marines, as well as a Demon and a Spectre at the back), and grab the goodies. In a small tunnel into the central section of the room you'll find the yellow keycard.

Now go back to the stairs and sprint to the bottom. Wait a bit, then sprint back up and around the central section to the right - a part of the wall up to a lit area of the wall will lower, and you can sprint onto it to access a secret area with some Imps, marines and... the Rocket Launcher! Excellent. Put it away for now, though; only use it in emergencies and/or at long range, as you do not get much ammo for it. Turn back to face the door to the previous room, and you'll notice a tunnel in the slime to the left big enough for you to go through. At the end (it's not far) you'll find a medikit and a Backpack. Throw the switch and then locate the off-colour wall section to the right of it. Open the wall and head through for some more goodies including two boxes of bullets and another chaingun. You can go up the lift for two medikits and access to the section of the level before you entered the dark bit up the stairs. But, turn around and go back through the slime tunnel. When you get to the wall you came in from, open it and sprint along the dark room to the right. The end wall will be doing the same thing as the one you just came through, and by getting on it you'll be able to access another secret area.

Down the stairs in here, kill the three Imps, and in the dark room at the end you can throw a switch on the pillar and make it drop to obtain a Soulsphere! This secret region is now exhausted, so go back up the stairs and through the dark room down to the door leading to the starting slime pit room. You'll now want to go all the way to the end where the other door is and go through it. Head through this entire next section of the level, killing all the marines, Imps and Demons as you go (be careful not to fall into the slime when you're outside). At the end, you'll find the blue keycard. Run onto it backwards - this will allow you to already be facing the enemies that come out of a wall section when the lights go out. Kill the four Imps, and head back through the level to the starting slime pit room.

You now have a choice. The raised platform allows you to access a secret wall section at the far end of the slime pit which leads to the secret exit of the level. If you do, simply follow that path to the end (you'll have to escape another monster ambush). If you go to the "proper" exit you'll not go to the secret level, not to mention having to tear through a lot more enemies (loads of shotgun marines and imps). This guide will assume you go to the secret level, as you go to level 4 once you complete it anyway. Head across the platform and open the wall at the end. Kill the Imps and the Demon, and head through to the exit door. Before throwing the switch, check the lift, as you can go up to a platform overlooking a previous section of the level which has a box of rockets on it.