Early D&D Dungeon Stocking Guidelines

These guidelines are from the time of the original campaign or shortly thereafter about how to stock a level map of a dungeon.

1974 OD&D Vol 3. "As a general rule there will be far more uninhabited space on a level than there will be space occupied by monsters, human or otherwise [...] It is a good idea to thoughtfully place several of the most important treasures, with or without monstrous guardians, and then switch to a random determination for the balance of the level. Naturally, the more important treasures will consist of various magical items and large amounts of wealth in the form of gems and jewelry. Once these have been secreted in out-of-the-way locations, a random distribution using a six-sided die can be made as follows: A roll of a 1 or 2 indicates that there is some monster there. [...] Roll again for every room and space. A roll of 1–3 in those rooms or spaces with monsters in them indicates some form of treasure is present. A roll of 1 in a room or space which is unoccupied indicates that there is some form of treasure there". 

Unguarded Treasures should be invisible, hidden behind a secret door or under the floor, locked in hard-to-open strong boxes with poison needles or deadly gas released when they are opened. (There are many variants of the above possible, and many other types of protection which can be devised.)

This results in about half the rooms being entirely empty, a bit less depending on how many special / unique encounters are placed. This original guidance is very similar to the later Moldvay Basic guidance from 1981: "Special monsters should be first placed in the appropriate rooms along with special treasures. The remaining rooms can be stocked as the DM wishes. If there is no preference as to how certain rooms are stocked, the following system may be used. d6, 1-2: monster, 3: trap, 4: special, 5-6: empty. Treasure d6,  1: yes, 2: if monster or trap, 3: if monster, 4-6: none."

Dungeon Geomorphs "Approximately 25% of the rooms and large spaces should contain monsters, treasures and other notable items. For every five such rooms there should be approximately one trap. Slanting passages, teleportation areas, slides and the like should be added sparingly thereafter -- one or two such items per level is a fair guideline". This has even a lower share of rooms with anything in them, about 60% of the rooms would end up empty.

Dungeon Geomorphs maps from 1976-77 are drawn in the "old" style and the instructions recommend that "approximately 25% of rooms and large spaces should contain monsters, treasure, and other notable items," even the early AD&D modules (G series, T1) and the sample dungeon level map in the DMG are in a very different style -- a lot more black space, a lot fewer rooms overall (the DMG sample level map has 38 numbered areas, several of which are more sections of hallway than rooms or chambers, and the dungeon level map in G1 has 33;  G1 has only 6 of 33 empty rooms -- less than 20%; about the same ratio as the moathouse dungeon in T1 which has 3 out of 18 empty rooms) and a lot fewer "maze" elements ("nested" rooms & corridors, long dead-end hallways, "nexus" points with lots of potential exits, etc.). At some point Gygax seems to have simply decided that he didn't like the maze-navigating aspect of early D&D. [...] The Storerooms map in CZ is consistent in style with pretty much every dungeon map Gary Gygax drew from 1978 on. (T. Foster Analysis)

Solo Dungeon Adventures, An article by Gray Gygax originally published in The Strategic Review Vol.1 No. 1 Spring 1975, has random room generation. It also has tables to populate them: d20 1-12 empty, 13-14 monster, 15-17 monster and treasure, 18 special or empty, 19 Trick/Trap, 20 Treasure. While Gygax said that he did not use it, as he found it faster to make stuff up, he expressed that this generated dungeons close to what he had in mind. Again, about 25% chance for a monster, and 60% empty rooms.

Gygax own Module B2, Keep of the Borderlands is likely the most played module of all times as it was included with the most sold version of the game. It  taught a generation of GMs by example how to design and populate a dungeon and had a much lower rate of empty rooms, only 11% of the rooms did not contain a monster.

From my own experiments, if one pre-populates some unique and interesting encounter areas as suggested, and then only uses the random determination for the remainder, fudging here and there to make things work, and filling some empty rooms with dungeon dressings, the OD&D / Moldvay system works pretty well. And the random placement of non-sensical monsters can be used to spur creative stories and explanations for it, so it makes sense. The latter is actually intense work.

Delta has a fantastic analysis of all of Gygax's own modules. According to this, Gygax real ratios were out of 6: 2 Monster, 1 Monster with Treasure, 1 Treasure, 1 Trick/Trap, 1 Empty. Unguarded treasure typcially was hidden or trapped, as advised above. 

Random Monsters 

OD&D on dungeon Lists: d6 to determine List to roll on

Level List 1        2        3        4        5        6

1                1-2    3-4    5        6
2                 1      2        3-4    5        6                               
3                         1        2        3-4     5        6
4-5                                1        2-3    4-5     6
6-7                                           1      2-4      5-6
8-9                                                    1-2     3-6
10-12                                                1        5-6
13+                                                              1-6
As Delta observes, this is very deadly, it would be better squish the ranges so the first three rows do not extend into their last listed category. You could run into dragons and vampires on level 3 here...

1. (d8): kobolds, goblins, skeletons, orcs, giant rats, centipedes, bandits, spiders
2. (d10) hobgoblins, zombies, lizards, gnolls, thouls, ghouls, berserkers, F2, W3, W4.
3. (d10) wights, giant hogs, giant ants, ochre jelly, giant snakes, and giant weasels, F4, F5,  W5, W6
4. (d10) wraiths, ogres, giant beetles, giant scorpions, lycanthropes, gargoyles, white apes, F6, C4, W7
5. (d12) trolls, wyverns, spectres, mummies, minotours, manticores, cockatrices, wyverns, hydra 6-8 heads, medusae, F8, W9, 
6. (d12) giants, hydra 9-12 heads, dragons, basilisks, gorgons, chimeras, vampires, spectres (really: balrogs), purple worm,  F9, C9*, W11*

*Typically each will be accompanied by from 1–4 apprentices (Enchanters/Evil Priests) and 1–6 body-guards (levels 4–6) fighter-type.

Other monsters to consider, depending upon the level and the surroundings: Giant Crabs, Giant Leeches, Giant Octopi, Crocodiles, Giant Squids, Sea Monsters,Nixies, Mermen, Griffons, Pterodactyls, Rocs, Invisible Stalkers, Cyborgs, Robots, Androids, Shadows, Dopplegangers

Number of Wandering Monsters Appearing: If the level beneath the surface roughly corresponds with the level of the monster then the number of monsters will be based on a single creature, modified by type (that is, Orcs and the like will be in groups) and the number of adventurers in the party. A party of from 1–3 would draw the basic number of monsters, 4–6 would bring about twice as many, and so on. The referee is advised to exercise his discretion in regard to exact determinations, for the number of variables is too great to make a hard and fast rule. There can be places where 300 Hobgoblins dwell. 

Replace Specter with Balrog in the last chart if your copy of the rules if from after the Tolkien estate forced TSR to remove direct Lord of the Ring references.

Since everything in OD&D except saving throws and attack rolls which used d20 was based on a d6, a consensus good default would be 1d6 for each type of monster of a table matching the dungeon level, so 1d6 hobgoblins on level 2. Most people also double the number with each level deeper, so 4d6 goblins woudl be an encounter for level 3.

Supplement 1: Greyhawk upgraded that to (bold are additions)

1. (d12) kobolds, goblins, orcs, skeletons, giant rats, centipedes, bandits/berserkers, spiders, stirges, dwarves/elves, gelatinous cube
2. (d12) hobgoblins, gnolls, zombies, ghouls, F2s,, W3s, W4s, T3s, T4s, giant toads/ants, bugbears, carrion crawler, removed berserkers to level 1, thouls, lizards
3. (d12) wights, F4s, F5s, W5s, W6s, T5s, T6s, harpies, wererats, giant snakes/spiders, giant weasels/hogs, ochre jelly remove ants
4. (d20) doppleganger, wraiths, gargoyles, ogre, C3s, F6s, W7s, T7s, shadows, giant beetles, giant scorpions, lycanthropes, carnivorous apes, owl bears, displacer beasts, blink dogs, phase spiders, giant ticks, will'o wisps, rust monsters; remove white apes
5. (d20) trolls, ogre magi, F8/Paladins, wyverns, spectres, mummies, druids, hell hounds, invisible stalkers, minotaurs, manticores, lammasu, cockatrices, W9s, W10s, T10s, salamanders, hydra 6-8 heads, umber hulks, giant slug;  remove medusae
6. (d20) giants, titans, hydra 9+ heads, dragons, golems, basilisks, gorgons, fire-breathing hydra, chimeras, vampires, hell hounds, F9/Paladins, efreet, beholders, evil high priests, W11s, T11s, lich, purple worm, rust monsters

Monsters

• Mimicry Monsters that appear as something else (harmless animals, rock, ochre jelly looks like Ogre, grey ooze like snake, black pudding like giant spider);
• Modified monsters (fire resistant mummies, skeletons that can hurl fingers like bolts, multi-headed giant that cannot be surprised);
• Monster combos (Medusa with Basilisk and cockatice pet, riding a gorgon; efreeti riding red dragon with salamander servants; frost giant riding white dragon; troll with magic spear on purple worm, ogre-mage on manticore, wizard with hell hounds, lord riding wyvern, evil high priest on chimera, thieves with bugbear guards, cloud giant riding a T-Rex, hero elf on lammasu)

Tricks and Traps 

OD&D: The fear of “death,” its risk each time, is one of the most stimulating parts of the game. It therefore behooves the campaign referee to include as many mystifying and dangerous areas as is consistent with a reasonable chance for survival (remembering that the monster population already threatens this survival). For example, there is no question that a player’s character could easily be killed by falling into a pit thirty feet deep or into a shallow pit filled with poisoned spikes, and this is quite undesirable in most instances.

Mapping Confusion

• Dead ends
• Sliding Walls, Blocks, rotating floors
• Diagonal passages that are difficult to correctly map
• Natural Caves and Caverns with varying width, direction that are near impossible to map
• Teleporters / Transporters (intra- or inter level), one way or two-way
• Space distortion corridors or stairs, sliding rooms
• False doors
• Secret Doors; one-way secret doors (only open from one side), two-way secret doors (depending how you open lead to two different areas); often as escape hatch for evil NPCs or bosses.
• Locked doors or stuck doors [all doors were swollen/hard to open by default, opening normally for monsters, another item that strains credibility]
• Doors that open only from one side; intermittently; for certain classes, alignments, monsters, with password; that allow traffic only in one direction, have (malign) intelligence
• Stairs up or down, Slides down, Stairs turning to slides (with monster at end), Chimneys / Shafts, False Stairs, Collapsing Stairs
• Steps which lead up a short distance, but then go downwards for at least two levels, with the return passage blocked by bars or a one-way door
• Elevators / Sinking Rooms (one or more levels up or down); fake elevators (appear only so)
• Slanting passages (lead players unwittingly onto another level), also combine with elevators, stairs [Note that this is a stupid feature that would not work. To have a decline that is imperceptible, these passages would need to be miles long]. 

Traps

• Pit traps, open, covered, with spikes, with poison spikes, closing, with secret door inside them, and then other traps behind those..., with teleporters
• Arrow/Spear traps, maybe poisoned
• Pendulum, ball, scythe or blade traps
• Gas, obscuring vision, blinding, sickening, weakening, slowing, deadly poison; inducing sleep, fear, confusion, memory loss, or strength
• Vents spewing acid spray, fire, gas
• Portcullises, falling blocks, collapsing ceiling
• Room traps: ceiling lowers to crush, flooding, floor collapses
• Magical curse room trap: illusion, mind-control, greed-inducing or geas
• Lightning bolt (interestingly, no explosions or fireballs!)
• Vegetation that holds, entwines, moves, flails with branches, makes noise, hurls missiles, or emits dangerous gases

Wondrous Items to muck around with

• Pools: random permanent stat increase/decrease, talking/oracle, transporter/teleporter, converting coins; granting wish; benefits often limited to once.
• Statues that do nothing, point somewhere, recite a meaningless poem, a clue or riddle, shout alarm, offer a real or false map, attack, attack and pursue etc.; Status with a missing piece that activate once it is found and replaced (to do the above or serve for a time)
• Levers, buttons, dials that damage mover, teleport, release traps or that give access to treasure, boni, items or open (secret) doors, pits or slides somewhere
• Cursed items, that cause bickering, attract monsters, shrink, reduce alertness
• Objects that change user or mover (stats, class, alignment, into monster, gender, level, mind)

In the DMG he realized you can really combo any object and magical effect and so listed first features (altars, arches, idols, fountains, pools, rooms, statues etc.) and then possible effects, like the ones listed above, plus: ages, animated, anti-magic, appears, disappears, asks, disintegrates, enlarges, reduces, shocks, petrifies, polymorphs, varies gravity, acts randomly, releases treasure, suspends animation, takes/steals, etc.

Hidden Treasure

• False bottom or hidden compartment in chests and cases 
• Invisible treasure
• Treasure is part of a monster (in gullet, valuable fur, tusks, embedded treasure in horns, claws hide)
• Force Fields protecting treasure 
• Maps (in a world of labyrinthine levels, quite useful), false maps...

Specific items from Greyhawk 

• Wishing well, grant a wish based on value of item tossed in - some just harbor a horrible monster
• The Living Room: Furniture comes to live and attacks wrapping, smothering, kicking, tripping
• The Great Bas-Relief Face: gives knowledge or increase, or save to not become a pimple on it (really Grazz't bound)
• Magic Item Forge with ultra powerful smiths, that sell at steep prices, no guarantee to work
• Hall of Gambling great items can be won but buy-in stakes are high, and the game is fixed
• Room that are the innards of a huge monster (mouth, stomach, intestine)

5th Edition Dungeon Stocking in comparison

5e Appendix A actually conserves most of the tables from the 1e DMG, and has quite detailed procedures to generate content. Why is nobody using that - it looks great? Probably because, compared to back then, now there are more than enough fleshed out dungeons. There is even a random dungeon generator at Donjon, that allows you to create 5e dungeons using these rules at the click of a button.