I've occasionally seen threads about this, using the Jabu-Jabu dungeon from OoT as an example...

The city is important since the inquisitor you work for JUST bought a vacation home there and does NOT want to blow it up till they've got their money's worth

I've got a civilisation that lives inside a colossal subterranean hibernating dragon. It goes without saying that they make everything from the dragon's tissues, but they also do their best to keep the dragon alive and fight off any threats to its wellbeing. Because if the dragon dies, so do the stunties.

You'd try to keep yourself in the lungs and esophagus if you wanted to live.

...

-Doors should be sphincters
-Traps are biological processes like a peristalsis corridors that crush occupants, glands that flood the chamber with digestive juices
- Many areas can only be accessed by swimming (don't ask)
-Monsters could be Immune system response like amorphous blob white blood cells, symbiotic organism used to aide digestion or smaller parasites that pray upon other nutrients ingested (like you) while hiding from the dungeons immune response.
-The body is a literal micro-biosphere so you could have a whole working ecosystem, even intelligent races who worship the creature and have become so important to homeostasis that the Immune system doesn't react to them.

Thing about colossal Crratures in Fantasy: you get a bit of leeway when it comes to breathable air.
Could say that most of it's internal tissues require oxygen, thus the lungs distribute air inaide of the body, or perhaps You, the Adventurer, have been granted the ability to breathe.
Or, it's a magic dragon so it doesn't matter.

I like this concept of a race that lives inside a giant dragon or creature. And it can be done many different ways, depending on the tone of the campaign. It can do anything from Lovecraftian "The creature doesn't know or care that they're in there," or, the opposite, "The dragon Swallowed these people to save them, so she could keep their race safe."
Either way, a race using biological material around them for building and armor sounds cool.

>Realistically
Stop. There not being breathable air is literally the least of your worries.
1. Giant creatures big enough to be dungeons could not exist in real life because of the square cubed law. They'd have to be in outer space, if anywhere. Underwater MIGHT work but even that is still pushing it.

2.Generally, being "Inside" a giant creature would be less dungeon and more Tunnel, assuming you get inside by being eaten. The GI tract goes from one end to another with no branching paths unless you can get "absorbed" somewhere along the line.

3. The gastric path for a creature that big would be hellish. Enjoy dropping straight into a pool of acid that melts stone. And even if you get out of the GI tract, the internals of a living thing are unbelievably harsh to external invaders. You'd be swarmed with all manner of immune responses from active attacking cells to deadly chemicals.

What if you had regular sized creatures and you suffered the effects of a shrink ray.

...

Only if you give them protective suits and/or a tiny submarine.

>muh shrink ray
irl.cs.ucla.edu/papers/right-size.html

Realistically a colossal creature would have to be created artificially anyway. Maybe the sea monster started out as a living aircraft carrier or something.

Remove number 1 but not either of the others. Living inside another creature is probably the most difficult thing possible because it is the only environment that is actively out to get you at all times.

Parasites, often considered "lesser" organisms are actually some of the most highly evolved in terms of their specific biological capacities and mechanisms. I mean, think about a liver fluke. It gets ingested, survives digestion, worms out into the blood stream and then somehow finds its way to the liver within hundreds of miles worth of pitch black blood vessels. Not only does it find the right vessel it avoids getting pumped into a capillary and being trapped. All while surviving attacks from the immune system.

I liked the part in Gears 2(?) where you fight your way through a gigantic worm. I think the enemies were giant parasites/antibodies.

>Living Aircraft Carrier
That actually gives me a ton of ideas.

>There was a war thousands of years ago
>ancient civilization was so advanced that they used biotech rather than crude mechanics
>Dragons, trolls, etc are all remnant weapons from the old war
>ancient Aircraft (Giant bird monsters) were launched from Engineered whales
>some still survive and/or were able to reproduce so that the party dungeon crawls through one.

Brilliant.

The "Keeping the race safe" concept sounds oretty nice to be honest. Magically make her immune system ignore them or something and it's actually a bit adorable.
>Dragoness sees this nearby village of smaller people under siege from monsters.
>She comes down to fight them off often, but it's draining to do so every day.
>Eventually realizes how significant of a size difference there is, and realizes how harmless the monsters are against her.
>Magic-protects the people in the village, and swallows them peacefully.
>They help keep her healthy, Clean, and, given her size, defend against invaders.
>She eats a little extra food so they have plenty to eat, Moves slowly/calmly to avoid "earthquakes," and watches over them.

Not a bad setting in all honesty. Players can find it and have a safe village where they can rest/buy stuff in the middle of the Dungeon crawl if needed.

>at the end of the 'dungeon' are a few fragments of the language once used to control it
>you can use them to summon magic airstrikes

Bruh, Magic Airstrikes would be an amazing reward.
Noting that for my campaign.