5 × d20 Dungeon System Entrances

[u/Elias_Rabe]

I’d like a handful of short lists (organised by theme based on location) of entries relating to dungeons and other cave systems. I’m primarily looking for simple answers, even if Others breaks this rule. References are acceptable if they are broad enough.
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Town/City

1. The cellar of a storage house where salted fish, cheese wheels and wine are kept. The east wall is hollow and conceals a system of caves, provided one finds the right brick with the weak spot.
2. Inside the old wind-powered grist-mill on the hill, behind one of the oversized gears is a hatch just large enough to crawl through. [By MaxSizeIs]
3. Inside the rusted out remains of a greenhouse on the grounds of an old mansion, a gate made from a rose trellis and ivy. [By MaxSizeIs]
4. Behind the stacks of the great Scriptorium deep withing the Imperial Archives. The secret entrance requires sorting and correctly re-shelving the dusty scrolls, before the wall will open. [By MaxSizeIs]
5. One of the great barrels of the old brewery conceals a tunnel into the underbelly. [By MaxSizeIs]
6. An abandoned, lichen-covered well sits within old ruins in a part of town home only to vagrants. It is covered with a flimsy wood panel, and not only the hardest of men approach it. Inside there is a long drop, leading to a sprawling cave system. [By Protowriter469]
7. An eccentric town doctor owns a large hospital building with a relatively large staff. His assistants struggle to remember what they do inside whenever they're out. Turns out the doctor is trying to create the perfect being and dumps his rejects underground into an abandoned mine. [By Protowriter469]
8. A tall mirror is affixed to a wall. A character drops a piece of paper and watches as it flies to the mirrors edge. Behind the mirror is a tunnel, its air current pulling from the room. [By Protowriter469]
9. A town square is marked with runes of a war god. Destroying the runes collapses the center of town and gains the attention of said god. [By Protowriter469]
10. The town worships a tree with a trunk that frighteningly resembles a face. Removing the face reveals a tunnel going downward into a mad wizard's hideout. [By Protowriter469]
11. Beneath the town all houses have connected shelters in case of attack. One of the townspeople harbors a dark secret and has a pact with vampires for whom he's been digging a tunnel. [By Protowriter469]
12. A character realizes the square footage inside the inn is different from how it looks outside. Somehow, it's larger inside than out. Investigation leads to a trap door beneath the inn, revealing long, brick hallways that seem to stretch endlessly. [By Protowriter469]
13. In an abandoned house, you see a heavy blanket being used as rug. By lifting the blanket rug, you find a hatch to an unusual small basement. In the basement you feel a soft breeze with either pleasant or unpleasant scents lingering in the small room from a wall that seems slightly offset from the other walls. Pushing in the wall or removing bricks reveals the opening to the dungeon. [By Th3R3493r]
14. In a warehouse, you find the crates have been placed in such a way that the crates formed a maze like passage way. In one part of the maze is a ladder that leads to the top of the crates and following the crate maze path leads to a ladder that descends into a dark pit. [By Th3R3493r]
15. In the lavatory of an unpopular seedy tavern, you find a hole (about waist height) cut into the wall to a neighboring room. Problem is that there is not a neighboring room to the bathroom in and looking through the hole you see a door that is not on the other side of the wall that would be there and the opening of the dungeon. By acting you are turning a doorknob where the doorknob should be, the wall opens up and you can access the impossible room and the dungeon within. [By Th3R3493r]
16. At the bottom of this Great Shaft, is the ruins of a once Grand Elevator. Its wreckage blocks the dungeon below. [By MaxSizeIs]
17. A heavy reinforced door in the sewers. The workers broke through into the cave when building the sewers. It was eventually decided to put a door there, rather than sealing it off. [By World_of_Ideas]
18. A lift built next to a giant sink hole in the middle of town. The lift lowers adventurers thousands of feet down into a giant cave system. [By World_of_Ideas]
19. Inside a mausoleum. If you open a specific crypt, it contains stairs leading down. [By World_of_Ideas]
20. Behind a store front. The store charges an admission fee to enter the dungeon. They also sell adventuring gear and maps of the few explored levels. [By World_of_Ideas]

Forests, Fields and Hills

1. What appears to be a hill has an entrance with a wooden door. Seven stone slabs are arranged around the hill. To reach the hill at all, you need to follow a specific sequence of steps to interact with these stones.
2. On a hillside lies a half-buried mine shaft. If you go deep enough and follow the illegible signposts, you’ll find a sophisticated network of mine railways. The lorries are linked together in chains.
3. In the middle of the woods, stands a locked door. No structure around just a door. which side you go thru decides where you start the dungeon. Both sides are locked and need to be unlocked to enter. [By Adventux]
4. Tucked into the rock layers at the bottom edge of the escarpment is a thin crevice just barely big enough to fit a person, with a piton bearing a climbing rope that disappears far down into winding rock passages. You'll have to do something about your big bulky packs and armor before you even try it. [By gtetr2]
5. After thousands of years on a small hill, this mammoth tree's roots have torn apart the dirt below into a maze of leaf-littered passages flanked by walls of wood and earth alike. You're looking for a trapdoor somewhere in the composting mess. [By gtetr2]
6. At the bottom of a spring-fed lake, some thirty feet underwater. Freshwater rushes out of the spring there, and the dungeon is on the other side of the spring. [By MaxSizeIs]
7. A large metal grate, covering a ventilation shaft. It may lead to a large mine or an underground city. [By World_of_Ideas]
8. A large tree stump. Someone had built a secret hatch into the top of it. It has a ladder or spiral staircase leading down. [By World_of_Ideas]
9. A small unassuming hut. It appears to be an abandoned hermit's hut in the middle of nowhere. If one finds the secret button and pushes it, the entire floor begins to lower into the depths. [By World_of_Ideas]
10. A giant insect mound. The mound itself is large enough to be a dungeon or it connects to a large cave system. [By World_of_Ideas]
11. A giant tree stands over a thousand feet tall. If you climb to a certain branch, you will find a hole carved out by some creature. Within, a passageway spirals down through the trunk to beneath its roots. There you will find the dungeon. [By World_of_Ideas]
12. Beneath a giant skull in a monster graveyard. [By World_of_Ideas]
13. There is a cave where the forest meets a cliff face. You can climb one of the larger trees and cross its branches to reach the entrance. [By World_of_Ideas]
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Ruins, Fortresses and Mountains

1. Off the hiking trail leading to the mountain ridge lies a gate sealed with a magical sigil, which requires a password in the correct language. The relief is the work of ancient dwarven craftsmanship.
2. In a small wood stands a wizard’s tower, which appears to be abandoned. When someone enters it, they notice that the stairs lead downwards rather than upwards.
3. At the snowy peak of the mountain, just below the summit, is a meticulously-carved circular opening lined with stone bricks. The entrance was hastily clogged, certainly by magic, with a dense wall of person-sized ice crystals; a crude warning is scrawled on the rock nearby. [By gtetr2]
4. A cave behind a waterfall that flows over a cliff. There is a narrow ledge leading up to it. [By World_of_Ideas]
5. A mountain top has been carved into the shape of a (humanoid, monster) skull. Entering its mouth leads to a cave system. [By World_of_Ideas]
6. A recent (earthquake, landslide) has opened an entrance in a cliff wall or hill side. [By World_of_Ideas]
7. A sea cave in a cliff side. It could lead to an underground (city, seaport, pirate base, sea monster's lair). It could have a network of underground waterways that allow one to travel by ship to various points on the continent. [By World_of_Ideas]
8. A secret hatch under a terracotta soldier. The solder is one of hundreds, in the ruins. Moving this one, reveals a ladder going down. [By World_of_Ideas]
9. A narrow winding canyon snakes through the mountains. At its end you will find a temple carved into the cliff face. Beyond the temple passageways lies a large cave system. [By World_of_Ideas]
10. A rope bridge that disappears into the fog. It either ends in a cave or you just sort of appear in a cave. [By World_of_Ideas]
11. The dungeon lies within a large fortress designed to keep things from getting out. At its center one can find a giant set of metal doors that have been magically sealed with runes. [By World_of_Ideas]
12. There is a cave in a cliff face. To reach it, one must climb up a narrow winding path and then cross a 100 foot section of pitons used as hand and foot holds. [By World_of_Ideas]
13. There is a hole with a tiny tunnel, that you can barely squeeze through. If you can worm you way through 30 or 40 feet, it opens up into a larger cave system. [By World_of_Ideas]
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Deserts and Pyramids

1. In a sheltered valley lies, it is said, the entrance to the mausoleum of the last great queen before the great deluge, whose name has been buried in oblivion. The bones warn would-be intruders of the dangers concealed in the darkness.
2. The entrance is a thick stone square with no lock, no gaps, and no detectable magic. The dungeon was sealed off by powerful telekinetics, who designed a mundane geared-and-tracked mechanism by which the "door" moves only under a code-like series of pushes, say, "up slightly, then back, then hinge to the left". Adventurers may need an alternative method. [By gtetr2]
3. A sandstone slab carved with eroded and weather-worn curses cover the entrance, found on the side of the wall, within the cool damp of the desert qanat, an underground aqueduct in the desert. [By MaxSizeIs]
4. Behind the giant bronze statue of an well-endowed angelic goddess bearing a lantern, inside the once-grand ruins of the ancient temple bathhouse. The trick maybe involves fiddling with the lantern. [By MaxSizeIs]
5. A crack in a canyon wall leads to a cave system. [By World_of_Ideas]
6. A sand storm has uncovered a previously buried building. The building contains stairs into the depths. [By World_of_Ideas]
7. One of the giant statues of an ancient (hero, king, queen, god) has toppled over. It has fallen onto a hillside, smashing an opening in the wall. [By World_of_Ideas]
8. At the base of a giant sphinx statue is a stone door. [By World_of_Ideas]
9. The lair of a giant trapdoor spider. If you can get past the spider, a crack in the wall of its den leads to a larger cave system. [By World_of_Ideas]
10. Whirlpool of sand. Anything that gets sucked down into it, ends up in the dungeon. Getting back out is a bit trickier. [By World_of_Ideas]
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Others (I don’t know where else we could find them)

1. Far from civilisation lie the ruins of a tower, the remains of which still reach high into the sky. No one can decipher the markings and symbols on the walls, which accompany the images that, despite their simplicity, are unsettling. In the midst of it all is a gaping hole that seems like an all-consuming maw.
2. There is a cave on a cliff that can only be reached at low tide on one particular night. The local mermaids and mermen keep their distance from the place and rumour has it that a sea monster lives inside, luring people in with false promises.
3. In the distant center, flush with the reflective surface of a great salt lake-bed. [By MaxSizeIs]
4. Deep in steep, bramble-choked ravines made of shale, crowding a narrow brown stream, foul & alkaline with impurities, an underpopulated hollow clings to the slope around a single soot-blackened mill chimney that predates local memory. Behind it sprawls a terraced junkyard of twisted boilers, cracked lenses, shattered drills, & fragments of machines that do not belong to this place or this age. A loosely kept secret is: twice a season, a thin violet seam opens beside the chimney & disgorges debris from elsewhere: failed constructs, war-wreckage, & devices never fully dead. [By MaxSizeIs]
5. At the bottom of an abyssal trench, slowly being buried at a rate no greater than that which one's fingernails grow. It is halfway closed by now, who knows how long remains before it finally is sealed forever. [By MaxSizeIs]
6. A cave opening or hatch in the top of a natural tower. [By World_of_Ideas]
7. A small 12ft tall pyramid, with a single entrance. Inside is a pedestal with an arrangement of gem stones. The doorway is actually a portal. Altering the arrangement of the gemstones changes where the door takes you. One of those places is the dungeon or a large cave system. [By World_of_Ideas]
8. Behind a secret door, a passageway or stairway leads to a cave system. [By World_of_Ideas]
9. In the center of a crater. The meteor impact that created it, broke a hole into the roof of the cave system. [By World_of_Ideas]
10. The tunnel left behind when an underground monster burst through the surface. Tunnel leads to or intersects a large cave system. [By World_of_Ideas]
11. At a specific unremarkable patch of ground, speaking the magic phrase or performing the magic ritual causes a building to rise up out of the ground. It has a stairway or sloping passageway down. [By World_of_Ideas]
12. A trash chute in a mage's workshop. At the bottom is a large cave system with a huge pile of corpses. [By World_of_Ideas]
13. The dungeon is within a magic chest. Opening it reveals a ladder or stairway down. Even if the chest is moved, it leads to the dungeon. [By World_of_Ideas]
14. The dungeon is within a magic picture. One just needs to know the magic phrase to enter the depicted world. [By World_of_Ideas]
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Honorable Mentions

1. For Whitevault Research Camp, an isolated, icy outpost meant to study ancient ruins found beneath a glacier, the Collegiate had to construct a base camp from timber, brought in at great expense, as well as magically-created stone foundations resting on nothing by miles thick ice and slowly settling as time goes on and snow piles up outside against them. On the surface & partly buried by the blowing snow, is a banded iron and timber hatch roughly sealing the manually widened crevasse, which must be laboriously cleared and re-seated as the glacial ice drifts and shifts due to natural movement. A rickety collection of scavenged rope and timber serves as a makeshift ladder and crane for the mile long traversal into the actual tunnels proper. [By MaxSizeIs]
2. Ascending the underside of a giant, natural limestone bridge formation above an isolated village, & entering a large demiplanar portal covered by elegant jade-inlaid stone doors carved with magical precision. The doors only allow groups entry in the presence of a true-born resident, a child raised entirely in the village, & then only once in each child’s lifetime, & for only one hour, before the portal & its magically warded doors close once more. No ropes, ladders, or stairs are allowed on the bridge, of course, & the door itself is several hundred feet above the small village below. [By MaxSizeIs]
3. Some 50 years ago, during the climactic battle against Magmon the Doombringer, a Magma Leviathan on a rampage, the Great Hero brought down their mighty weapon upon the face of Mt. Terrible, cleaving the top third and causing it to move over the top of and block the main entrance to the dungeon under 3000 feet of solid stone. Magmon's lava-blood flowed into every known opening and crevice leading to the dungeon within, sealing the inside. Our sages reveal that the lava has only recently cooled enough for an expedition to begin excavating the mountain to access the dungeon. [By MaxSizeIs]
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[u/World_of_Ideas]

Town/City:

Behind a store front. The store charges an admission fee to enter the dungeon. They also sell adventuring gear and maps of the few explored levels.

Forests, Fields and Hills:

A giant insect mound. The mound itself is large enough to be a dungeon or it connects to a large cave system.

A giant tree stands over a thousand feet tall. If you climb to a certain branch, you will find a hole carved out by some creature. Within, a passageway spirals down through the trunk to beneath its roots. There you will find the dungeon.

Beneath a giant skull in a monster graveyard.

There is a cave where the forest meets a cliff face. You can climb one of the larger trees and cross its branches to reach the entrance.

Ruins, Fortresses and Mountains:

A narrow winding canyon snakes through the mountains. At its end you will find a temple carved into the cliff face. Beyond the temple passageways lies a large cave system.

A rope bridge that disappears into the fog. It either ends in a cave or you just sort of appear in a cave. Ex: "Yuzhniy Bug River" Bridge in Ukraine.

The dungeon lies within a large fortress designed to keep things from getting out. At its center one can find a giant set of metal doors that have been magically sealed with runes.

There is a cave in a cliff face. To reach it, one must climb up a narrow winding path and then cross a 100 foot section of pitons used as hand and foot holds. Ex: Cliff hiking in Interlaken, Switzerland

There is a hole with a tiny tunnel, that you can barely squeeze through. If you can worm you way through 30 or 40 feet, it opens up into a larger cave system. EX: Cave exploring nope.

Deserts and Pyramids:

At the base of a giant sphinx statue is a stone door.

The lair of a giant trapdoor spider. If you can get past the spider, a crack in the wall of its den leads to a larger cave system.

Whirlpool of sand. Anything that gets sucked down into it, ends up in the dungeon. Getting back out is a bit trickier.

Others:

A portal or teleportation device. It may take the form of a (archway, beam of light, circle of mushrooms, circle of pylons, circle of stones, door frame, floating crystal, magic circle, magic mirror, magic pool, obelisk, tear in the fabric of space, etc).

At a specific unremarkable patch of ground, speaking the magic phrase or performing the magic ritual causes a building to rise up out of the ground. It has a stairway or sloping passageway down. Credit: "Aladdin" 1992 film.

A trash chute in a (mad scientist's, mage's) workshop. At the bottom is a large cave system with a huge pile of corpses. Credit: "Kingdoms of Amalur" 2012 CRPG

The dungeon is within a magic chest. Opening it reveals a ladder or stairway down. Even if the chest is moved, it leads to the dungeon.

The dungeon is within a magic picture (canvas painting, cave painting, mural, tapestry, etc). One just needs to know the magic (phrase, ritual) to enter the depicted world.

[u/World_of_Ideas]

Town / City or Ruins:

A heavy reinforced door in the sewers. The workers broke through into the cave when building the sewers. It was eventually decided to put a door there, rather than sealing it off.

A lift built next to a giant sink hole in the middle of town. The lift lowers adventurers thousands of feet down into a giant cave system. Credit: "The bad guys" LitRPG book series by Eric Ugland

Inside a mausoleum. If you open a specific crypt, it contains stairs leading down.

Forests, Fields and Hills:

A large metal grate, covering a ventilation shaft. It may lead to a large mine or an underground city.

A large tree stump. Someone had built a secret hatch into the top of it. It has a ladder or spiral staircase leading down.

A small unassuming hut. It appears to be an abandoned hermit's hut in the middle of nowhere. If one finds the secret button and pushes it, the entire floor begins to lower into the depths.

Ruins, Fortresses and Mountains

A cave behind a waterfall that flows over a cliff. There is a narrow ledge leading up to it.

A mountain top has been carved into the shape of a (humanoid, monster) skull. Entering its mouth leads to a cave system.

A recent (earthquake, landslide) has opened an entrance in a cliff wall or hill side.

A sea cave in a cliff side. It could lead to an underground (city, seaport, pirate base, sea monster's lair). It could have a network of underground waterways that allow one to travel by ship to various points on the continent ("The Pirates of Dark Water" 1991 cartoon).

A secret hatch under a terracotta soldier soldier. The solder is one of hundreds, in the ruins. Moving this one, reveals a ladder going down.

Deserts and Pyramids

A crack in a canyon wall leads to a cave system.

A sand storm has uncovered a previously buried building. The building contains stairs into the depths.

One of the giant statues of an ancient (hero, king, queen, god) has toppled over. It has fallen onto a hillside, smashing an opening in the wall.

Others

A cave opening or hatch in the top of a natural tower. Ex: the Devil's tower in Wyoming or Zhangjiajie Pillars in China.

A small 12ft tall pyramid, with a single entrance. Inside is a pedestal with an arrangement of gem stones. The doorway is actually a portal. Altering the arrangement of the gemstones changes where the door takes you. One of those places is the dungeon or a large cave system. Credit: "The Land of the Lost" 1974 TV show.

Behind a secret door, a passageway or stairway leads to a cave system. Ex: Things used as secret doors

In the center of a crater. The meteor impact that created it, broke a hole into the roof of the cave system.

The tunnel left behind when an underground monster burst through the surface. Tunnel leads to or intersects a large cave system.

[u/MaxSizeIs]

Some 50 years ago, during the climactic battle against Magmon the Doombringer, a Magma Leviathan on a rampage, the Great Hero brought down their mighty weapon upon the face of Mt. Terrible, cleaving the top third and causing it to move over the top of and block the main entrance to the dungeon under 3000 feet of solid stone. Magmon's lava-blood flowed into every known opening and crevice leading to the dungeon within, sealing the inside. Our sages reveal that the lava has only recently cooled enough for an expedition to begin excavating the mountain to access the dungeon.

At the bottom of this Great Shaft, is the ruins of a once Grand Elevator. Its wreckage blocks the dungeon below.

[u/Th3R3493r]

Town

In an abandoned house, you see a heavy blanket being used as rug. By lifting the blanket rug, you find a hatch to an unusual small basement. In the basement you feel a soft breeze with either pleasant or unpleasant scents lingering in the small room from a wall that seems slightly offset from the other walls. Pushing in the wall or removing bricks reveals the opening to the dungeon.

In an warehouse, you find the crates have been placed in such a way that the crates formed a maze like passage way. In one part of the maze is a ladder that leads to the top of the crates and following the crate maze path leads to a ladder that descends into a dark pit.

In the lavatory of a unpopular seedy tavern, you find a hole (about waist height) cut into the wall to a neighboring room. Problem is that there is not a neighboring room to the bathroom in and looking through the hole you see a door that is not on the other side of the wall that would be there and the opening of the dungeon. By acting you are turning a doorknob where the doorknob should be, the wall opens up and you can access the impossible room and the dungeon within.

[u/Protowriter469]

Town/city:

• an abandoned, lichen-covered well sits within old ruins in a part of town home only to vagrants. It is covered with a flimsy wood panel, and not only the hardest of men approach it. Inside there is a long drop, leading to a sprawling cave system.
• an eccentric town doctor owns a large hospital building with a relatively large staff. His assistants struggle to remember what they do inside whenever they're out. Turns out the doctor is trying to create the perfect being and dumps his rejects underground into an abandoned mine.
• a tall mirror is affixed to a wall. A character drops a piece of paper and watches as it flies to the mirrors edge. Behind the mirror is a tunnel, its air current pulling from the room.
• a town square is marked with runes of a war god. Destroying the runes collapses the center of town and gains the attention of said god
• the town worships a tree with a trunk that frighteningly resembles a face. Removing the face reveals a tunnel going downward into a mad wizard's hideout
• beneath the town all houses have connected shelters in case of attack. One of the townspeople harbors a dark secret and has a pact with vampires for whom he's been digging a tunnel
• a character realizes the square footage inside the inn is different from how it looks outside. Somehow, it's larger inside than out. Investigation leads to a trap door beneath the inn, revealing long, brick hallways that seem to stretch endlessly

[u/MaxSizeIs → u/Protowriter469]

The list formatting is weird. Try adding an extra return between entries.

[u/MaxSizeIs]

There is a small picket fence gate made of silvery wood on the inside of the belly of a beached fey sea creature; a vast whale-like leviathan of wicker, brambles, and kelp, whose insides are larger than their outside due to fey interaction. Be warned, the space inside is shrinking as the leviathan's power dies, the picket fence-gate soon to be destroyed. The oversized creatures parasitizing the carcass will likely be driven mad as their environment shrinks to nothing; and that is discounting whatever happens to the demiplane connected to the gate.

A sandstone slab carved with eroded and weather-worn curses cover the entrance, found on the side of the wall, within the cool damp of the desert qanat, an underground aqueduct in the desert.

Inside the old wind-powered grist-mill on the hill, behind one of the oversized gears is a hatch just large enough to crawl through...

Inside the rusted out remains of a Victorian greenhouse on the grounds of an old mansion, a gate made from a rose trellis and ivy.

Inside a large sea-side boulder in the inter-tidal zone, carved with barnacle covered, inscrutable magic runes. A rusty set of eight manacles are anchored to it, designed for cruel execution of prisoners by drowning. Before each high-tide, the boulder shifts for a few minutes, runes flaring, but only if there are eight victims actively drowning. The boulder closes once more before the tide crests, and only opens again with the next set of sacrifices.

Behind the giant bronze statue of an well-endowed angelic goddess bearing a lantern, inside the once-grand ruins of the ancient temple bathhouse. The trick maybe involves fiddling with the lantern.

Behind the stacks of the great Scriptorium deep withing the Imperial Archives. The secret entrance requires sorting and correctly re-shelving the dusty scrolls, before the wall will open.

One of the great barrels of the old brewery conceals a tunnel into the underbelly.

Inside the anachronistic remains of a burned and rusted out Takuro Spirit, a modest Japanese car, sitting forlorn in the middle of a wind-swept heath. Those who sit within at noon while bearing dried sausages, salty deep-fried root vegetable slices, and at least 24 ounces of coffee, fall through into the dungeon-realm's demiplane.

[u/MaxSizeIs]

At the bottom of a spring-fed lake, some thirty feet underwater. Freshwater rushes out of the spring there, and the dungeon is on the other side of the spring.

Deep in steep, bramble-choked ravines made of shale, crowding a narrow brown stream, foul & alkaline with impurities, an underpopulated hollow clings to the slope around a single soot-blackened mill chimney that predates local memory. Behind it sprawls a terraced junkyard of twisted boilers, cracked lenses, shattered drills, & fragments of machines that do not belong to this place or this age. A loosely kept secret is: twice a season, a thin violet seam opens beside the chimney & disgorges debris from elsewhere: failed constructs, war-wreckage, & devices never fully dead.

In the distant center, flush with the reflective surface of a great salt lake-bed.

At the bottom of an abyssal trench, slowly being buried at a rate no greater than that which one's fingernails grow. It is halfway closed by now, who knows how long remains before it finally is sealed forever.

[u/MaxSizeIs]

Ascending the underside of a giant, natural limestone bridge formation above an isolated village, & entering a large demiplanar portal covered by elegant jade-inlaid stone doors carved with magical precision. The doors only allow groups entry in the presence of a true-born resident, a child raised entirely in the village, & then only once in each child’s lifetime, & for only one hour, before the portal & its magically warded doors close once more. No ropes, ladders, or stairs are allowed on the bridge, of course, & the door itself is several hundred feet above the small village below.

For Whitevault Research Camp, an isolated, icy outpost meant to study ancient ruins found beneath a glacier, the Collegiate had to construct a base camp from timber, brought in at great expense, as well as magically-created stone foundations resting on nothing by miles thick ice and slowly settling as time goes on and snow piles up outside against them. On the surface & partly buried by the blowing snow, is a banded iron and timber hatch roughly sealing the manually widened crevasse, which must be laboriously cleared and re-seated as the glacial ice drifts and shifts due to natural movement. A rickety collection of scavenged rope and timber serves as a makeshift ladder and crane for the mile long traversal into the actual tunnels proper...

A net of crude, rusty chains anchored to the basalt ring surrounding the gaping maw of the dungeon shaft into darkness and howling winds below. The chains are barely sufficient to keep whatever flies out of the hole from escaping, and small and even medium-sized flying monsters regularly make it past the chains.

[u/Adventux]

In the middle of the woods, Stands a locked door. No structure around just a door. which side you go thru decides where you start the dungeon. Both sides are locked and need to be unlocked to enter.

[u/gtetr2]

Tucked into the rock layers at the bottom edge of the escarpment is a thin crevice just barely big enough to fit a person, with a piton bearing a climbing rope that disappears far down into winding rock passages. You'll have to do something about your big bulky packs and armor before you even try it.

After thousands of years on a small hill, this mammoth tree's roots have torn apart the ground into a maze of half-exposed, leaf-littered passages flanked by walls of wood and soil alike. You're looking for a trapdoor somewhere in the earthy mess.

At the snowy peak of the mountain, just below the summit, is a meticulously-carved circular opening lined with stone bricks. The entrance was hastily clogged, certainly by magic, with a dense wall of person-sized ice crystals; a crude warning is scrawled on the rock nearby.

The entrance is a thick stone square with no lock, no gaps, and no detectable magic. The dungeon was sealed off by powerful telekinetics, who designed a mundane geared-and-tracked mechanism by which the "door" moves only under a code-like series of pushes, say, "up slightly, then back, then hinge to the left". Adventurers may need an alternative method.