Lower level parties sally forth to the dungeon with a healthy concern for logistics - food, shelter, water, light, etc. Failing to stock up can mean their doom. Later, advanced magic easily makes up for a lack of preparation - or is outright superior! This can make for an unwelcome switch in play style for those who enjoyed the tension of their earlier forays.
In most dungeon games this change in play style is inevitable. Like it or not, it is part of the experience. It allows the party to take on a grander scale of adventure after proving themselves in the muck. Still, it feels like a sad error to abandon the grim lessons of earlier adventures entirely. Players who took these to heart do not want to see them drift into obsolescence. Those who did not may fail to even respect the threat of the dungeon.
To this point, I have assembled some methods to keep dungeons dangerous. The point here is not to turn a dungeon into the Tomb of Horrors but to show how a dungeon can be drawn up in a way that preserves its aura of mundane peril despite level.
Silent But Deadly
The flame of torches and lanterns can reveal deadly oxygen-displacing gases. Make their light flicker when they get close to an otherwise undetectable pool of suffocating or poisonous gas. Have it snuff out when they would need to make saves. Infravision and light spells have no flame to reveal this, so parties reliant on such powers find themselves doomed by their own superiority.
Obviously, this gets tricky if the pool of gas happens to be flammable. Consider that it is still better to find that out at its edge rather than while casting a fire spell within it later. A torch or lantern that merely bursts or changes color near a concentration of flammable gas can be a lifesaver for an observant party.
Lurkers in the Darkness
It is dark. You have been eaten by a grue. An antidote to all the fancy races with dark vision and their annoying players who have to tell you about it every time. Sure, they can see the grue, but will wish they hadn’t. A hazard easily fixed by a torch or lantern, but also amenable to light spells. Bold and greedy merchants in town could mark up their torches and advertise them as “grue repellent.”
This doesn't need to be as exotic as a grue. Animals and primitive monsters may be repelled by fire itself, to the point they avoid parties carrying open flame. This is not so easily outclassed by light spells.
The Tight Squeeze
They call it a dungeon crawl, so why aren't they crawling? Heavy armor and huge weapons are great until you have to squeeze through a tiny cave tunnel. Then, it is time for the lowly, skinny, flexible goblin to shine. A party that would beat the gods themselves on the field of battle is just a wiggly, screaming meal when pinned between two solid stone cave surfaces.
Just getting stuck may arguably be just as bad - the recent Nutty Putty cave incident being an example. This awful fate should bring a little tremor to the heart of the bravest adventurer, and at the very least make them consider hiring a local guide to keep them out of the worst passages.
This is an unglamorous way to go and it makes sense to give warnings. The tooth-marked skeletons of an early party, trapped in their rusting armor should do. I'm sure you can think of something better.
Spells like passwall and stone to mud, divination, etc. can avoid this, but this is at least a cost.
