Time, water, and the dungeon

Dungeons have a problem. The problem of entropy!

Metal rusts. Various bios degrade. Water seeps in, rotting and ruining things, warping wood, destroying mechanisms, bringing silt. Air itself is corrosive to most substances in the long term, hence the word "oxidation". Absent any countervailing forces, everything that we want in a dungeon decays.

Modern steel locks may seize up or jam quite fast in moderate humidity, especially if unused for long periods of time. I can't imagine how much faster it would happen for a tiny poison needle trap, embedded inside a lock, made with pre-modern technology.

Over the course of a few years (months somewhere moist, decades in an arid clime), locks, traps, hinges, volatile liquids, and stringed weapons will fall into ruin.

Over the course of a few decades (years in the damp, centuries in a desert), things like leather, plaster, armour, weapon blades and hafts, and ropes will join them.

Over longer periods, planks, leather, and textiles, and more solid metalwork will fall before entropy. Statuary, ceramics, and tiles might survive... except that the construction around them won't. Debris blows in - dirt, sand, dust, leaf litter. Walls get eroded by groundwater. Earth tremors rattle the place. Ceilings collapse. An underground structure anywhere near the water table gets infiltrated by groundwater produced by seasonal rain. Intermittent saturation makes the foundation give way.

Entropy! It means that on the face of it, the idea of "the ancient dungeon, recently rediscovered" fails the verisimilitude sniff test.

And then there's the really short-lived stuff: monsters. All organic life is fragile in the long term. Populations are fragile in the longer long term. A "bottle" ecosystem is very difficult to set up, let alone one with large animals, let alone big dangerous predators, let alone underground, let alone without deliberate design.

Deathless monsters only go so far; having local monsters wander in to 'lair' is only a partial solution; sometimes you really want it to be an ancient megastructure only recently breached. In particular we'd like there to be some monsters that only occur in the dungeon, lost to time. At some point a monster that's familiar to everyone just becomes an animal.

But we want forgotten languages, ancient traps, and primeval monsters! The detritus of a forgotten civilisation like museum pieces preserved under glass, inside a still-intact sprawling architectural masterpiece! What's to be done?

I think the possible solutions mostly fall under three categories:

1. Meticulous caretakery
2. It's just magic
3. Encysted outside time

Meticulous caretakery

Sapient creatures are the ultimate force of negentropy. Nobody's surprised to see pristine valuable materials and oiled clockwork traps when there are people around. Or people-like entities. Here's a few popular variations.

• The dungeon is isolated and maintained by a cut-off relict population of dangerous monstrous sapients, at least until recently
• The dungeon is leaking out of a nightmare dimension or a mythic underworld, with an innate sapience behind it providing direction
• The dungeon is a work of deliberate artifice, actively maintained - perhaps as entertainment for the viciously rich, or a gauntlet to prove religious valour
• The dungeon is filled with autonomous workers: golems or gargoyles or spell-constructs or immortal dwarves or undead servitors or elementals or generations of replenishing vat-bred monstrous servitors
• The dungeon is a delicately balanced ecosystem fought over by a set of sapient factions each with different but partly overlapping self-interests in maintaining and replenishing certain aspects of the dungeon

It's just magic

I tend to think of this as the default, when things are otherwise unexplained. That said, just because it's magic shouldn't mean it's unexamined, or that there should be no detectable signs of that magic.

Magical maintenance can be infinitely varied in form, function, and rationale. The simplest is an enchantment which silently cleans, scours, replaces and rebuilds as necessary. Further up the scale, Acererak's demonic housekeeping staff linger in the ethereal plane between delving parties. But it can get as odd as you can imagine.

Some games like it to not be magic, but elaborate systems of high technology. It has to be pretty advanced: at our own earth tech level there are plenty of things we can't make last all that long outside of exceptionally arid environments.

Encysted outside time

Very effective if done well, especially as a setting-wide feature, in this solution the dungeon has been preserved in stasis until recently.

It means the dungeon can be arbitrarily large, old, and well-populated: things pick up wherever they left off however long ago. The (magical) stasis itself in turn needs a reason within the setting, and in my opinion needs to be readily discoverable by player characters, but this is not difficult to achieve. And there are all sorts of knock-on effects and variations you can have fun with.

And so

Entropy! Don't let the gradual dissolution of all things ruin your fantasy world's verisimilitude.