And yet this isn’t actually the case.
Part of the challenge of old-school D&D lies in the subversion of this expectation. Sometimes you’ll tackle unexpectedly weak opponents that’ll drain your resources without giving you much reward. And more importantly, sometimes you’ll run into enemies much stronger than you are. At this point, you’d better be ready to get lucky, use up precious one-shot magic items, run away or die.
The Moldvay Red Box deals with this in a relatively tame fashion (p.B29):
“A monster’s level is only a guide, and a monster could be found anywhere in a dungeon, whatever the level. However, as a general rule, it is useful to limit monsters to 2 dungeon levels higher or lower than their hit dice. When monsters are encountered on dungeon levels less than the monster’s level, there should be fewer monsters than normal. And when monsters are met on dungeon levels greater than the monsters’ level, there should be more monsters than normal. EXAMPLE: A 4th level monster might be found anywhere in dungeon levels 2 through 6, but it is not likely to be found on the 1st or 7th levels except one at a time (on the 1st level) or in large numbers (on the 7th level or below).
OD&D is more precise, presenting a matrix for determining which level’s random encounter table you should use. For example, on the second dungeon level, you’d roll 1d6. Roll a 1, it’s a first-level monster. Roll a 2, it’s a second-level monster. Roll a 3 or a 4, it’s a third-level monster. Roll a 5, it’s a fourth-level monster. And if you roll a 6, it’s a fifth-level monster.
And then you get the Monster & Treasure Assortment, at which point all bets are off.
This handy old supplement provides lists of 100 monster encounters at each level from One through Nine, making it quick and easy to fill in a dungeon level. And there’s a broad spread of nastiness available at each level, with some monsters being much stronger than you’d expect to find. Let’s see what over-the-top possibilities can be found here:
Level One: 1 carrion crawler, 1 gelatinous cube, 1 giant black widow spider, 1-2 third-level M-Us, 1-2 third-level clerics, 1-2 third-level thieves, 1 fourth-level fighter
Level Two: 1 wyvern, 1 werebear, 1 owlbear, 1 wraith, 1-4 giant draco lizards, 1 sixth-level M-U
Level Three: 1 troll, 2-5 fifth-level priests, 1-2 seventh-level priests
Level Four: 1 wyvern, 1-2 stone giants, 1-2 werebears, 1-2 trolls, 1-3 seventh-level priests, 1-2 eighth-level fighters
Level Five: 1 eight-HD green dragon, 1 black pudding, 1-3 seventh-level thieves, 1-2 eighth-level clerics, 1 ninth-level thief, 1-4 ninth-level M-Us, 1 eleventh-level M-U
Level Six: 1 seven-HD black dragon, 1 seven-HD blue dragon, 1 Type I demon, 1-4 hill giants, 1-3 frost giants, 1-2 fire giants, 1 nine-headed hydra, 1 black pudding, 1-2 ninth-level fighters, 1-3 tenth-level M-Us
Level Seven: 1 Type III demon, 1 Type II demon, 1-2 frost giants
Level Eight: 1 ten-HD red dragon, 1 Type V demon, 1 Type IV demon, 1 thirteen-headed hydra, 1 purple worm, 1-3 tenth-level thieves, 1 twelfth-level thief, 1-3 tenth-level M-Us, 1 twelfth-level M-U
Level Nine: 1 eleventh-level gold dragon, 1-2 ten-HD red dragons, 1-2 cloud giants, 1 twelve-headed hydra, 1-2 purple worms, 1 giant slug, 1-2 eleventh-level fighters, 1 thirteenth-level M-U
As you can see, there are some freakishly powerful adversaries to be found in the Monster & Treasure Assortment. There are three distinct 6-hit die entries on Level Two. Level Four gives us 9-hit die stone giants, and Level 5 gives us the 10-hit die black pudding, our first dragon, up to four ninth-level Magic-Users and one shockingly puissant eleventh-level M-U!
Clearly this isn’t the modern 4e “fair and balanced” monster table. The old-school dungeon is much wilder and less predictable. When you delve into such a dungeon, watch your back and be ready to run!