Way back in issues 120-something of the venerable Dragon Magazine there was a brief piece on "Tucker's Kobolds" wherein an example was provided of devious tactics using kobolds and how a traditionally wimpy D&D monster could be turned into a horror with a little planning and knowledge of their environment. Over the years many a DM has run with that advice and magnified the cleverness, deviousness, and ability of kobolds and other little baddies to a ridiculous extent with little ones dwellign in a death trap that would make the villain of the SAW films blush.
Most folks ignore the sneakiest trap of them all: A dungeon (or sub-level thereof) built in scale to smaller humanoids. The sneakiest trap of the them all that will provide small humanoids an amazing advantage over PC parties is the low ceiling, narrow corridor, and tiny doorway. Life is hard on a 6' tall warrior in plate armor with a two-handed sword in 2.5' wide corridor. A doughty dwarven trollslayer (at 4'3" tall) and his +2 double headed flaming batttleaxe is at a disadvantage in a 4' high chamber. A 1" wide door presents a durable obstacle and choke point against man sized PCs with backpacks full of gear and loot.
Game mechanics should be tough on warriors caught in tight spaces, real life sure was. Roll d12's to hit instead of D20's when the space is too tight if not outright prohibition of the use of some weapons.
Halve damage rolls for folks that must squeeze,stoop, and crawl. Reduce AC or let the little baddies roll a bonus die against big folk crammed in little dungeons (1d20+1d8 to hit possibly).
Some clever players may realize having gnomes and halflings in the party (or similar wee ones) would provide the party with some flexibility in cordiors where Earl DeDuke must stoop and Change-o the Conjuror would wish his staff of wizardry could be bent in half. Putting monsters in the best environment for the monsters themselves will still prove to be the sneakiest trap of them all,