I've got an old issue of PC Format lying around (February 1995) which has got a feature in it on how to create Doom levels with DEU. And they'd also got Sandy Peterson to contribute ten tips on how to make a good level. I suspect some of them may have been polished by the magazine's editorial team as the idiom is very British, but it seems to match quite well with the info in Vermil's scans. And so here they are:
1. Let the player see stuff in action
If you have a button that builds stairs, or a trigger that raises a bridge or some similar device, point the player towards the event so that he can watch it take place. Occasionally you'll want these things to occur when he's not around, to surprise him, but generally it's best to show him that he's just accomplished something. (A sense of achievement is important for the player's morale - or he might feel inclined to give up.)
2. Use real architecture
Look at photographs of the Arc de Triumph, Buckingham Palace, etc. Steal the structures of these to make your own stuff look cooler. One important architectural tip - architects see buildings in terms of spaces, not walls. Once you start creating your rooms as spaces, they'll look a lot better, and be better to play in. It's amateurish to play lines down as if you were making a D&D dungeon, so try to rise above this tendency.
3. Tell the player he's walking into a trap
If all the doors suddenly open and a million imps pour out, that's not really scary. What is scary is opening a door and seeing a dark room, empty but for a rocket launcher. Now, you know that when you grab it, all Hell is going to break loose: so you get nervous. Note that the proximity of Barons o' Hell is almost always marked with a big sculpture of same. That way the player feels jittery well before said Baron emerges. [tip is illustrated with a screenshot of the illuminated Baron wall just before the first Baron in E2M4]
4. Always mark your secret doors
Every secret door in Doom is marked. Often the mark is very subtle, but it's there. Sometimes a light is shining on the door; sometimes there's a window into the secret room; in a couple of places there's even an arrow on the automap pointing to it; or maybe it's at the end of a dead-end corridor. In Wolfenstein it was a major pain in the ass to run along all the walls, bumping them for secret doors: Doom did away with that, and now logic rules.
5. Have a theme for each level
Each level of Doom has a play theme. The Unholy Cathedral (E3M5) is a huge room surrounded by a corridor. You teleport through all four corners to accomplish the coolest stuff. Mount Erebus (E3M2) [yes, he got the level name wrong] is all nooks and crannies, with monsters hiding behind boulders. Doom 2's Barrels o' Fun level is all about obstacle courses. You get the picture: decide the theme before you start designing, and the ideas will come to you.
6. Don't run out of ammo
It's more fun to go down shooting than punching. If you want to make the level tougher, add more or meaner monsters - don't take away ammunition. It's okay to make the player hunt for one particular type of ammo, as long as you have a place where he can get hold of it. But make sure you don't let him run dry. Also, be nice and generous with healing etc; make sure there's plenty of it scattered about.
7. Every switch is a reward
A player should always want to flip a switch or push a button, so doing this should never be a complete disaster. Don't have buttons which, when pushed, just kill you dead: all should have some redeeming feature. If pulling the switch unleashes sixteen Cyberdemons in a circle around the player, then by golly make sure that, once he kills the Cyberdemons, he gets all seven powerups or something as a reward for his pains.
8. It's cool to be dark, but it's not cool to be blind
Light your levels intelligently. Big flashing lights are good for two purposes - to draw the player's attention to something you want him to notice, or to draw his attention away from something which will make him soil his Y-fronts (horrid, lurking monsters). A dark place lit by bright strobes can be a very effective area, but don't overdo it by making the place too large: small is claustrophobic, and claustrophobic is scary.
9. Plagiarise shamelessly
If you see an idea you liked in someone else's levels, by all means steal it. By the time you're done modifying it, it will hardly be recognisable. You don't want the player to recognise it because if he does, he'll instantly know what to do about it, and we don't want that, do we? For example, the huge number of rooftop leaps in Doom 2's level 13 (Downtown) exist because I was very impressed with level E1M7's leap from pillar top to slime pit in Doom 1.
10. No stop 'til bosstime
It's good to have tough puzzles, but the player should never feel he's banging his head against a wall. Take level 8 (Tricks 'n' Traps) in Doom 2. This level has a bunch of thorny puzzles, but most players feel that they're progressing: they keep coming back because they convince themselves that they were almost there. If you do have a puzzle that requires a brilliant solution, make sure the player doesn't have to solve it to finish the level.