GURPS Dungeon Fantasy Setting: Caverntown

[Bruno]

Assuming the rough default of a 1.25mi (2km) on a side space for Caverntown, and not fiddling with the ratio of height to x-y dimensions, how high is the dome ceiling of the cavern?
I'm noodling around with making a model of caverntown in Blender, but PCs will inevitably want to fly or unleash a giant flying monster or both. How much clearance over houses and how high you can get and make guards/PCs get big range penalties to hit you is interesting (And as always, falling damage! PCs will find a way to take falling damage even on the arctic tundra. True story).

Height also indirectly controls how tall the buildings are going to get. If e.g. the ceiling is 30', and people have a roof patio, they're never getting more than 2 floors (including ground floor, not counting on top the roof) "above ground" and you'll not have a lot of space to maneuver over parapets and any roof-top gazebos. It will also cramp the style of the druids trees.

If the ceiling is 600' you can get some classy aerial dogfights going, and PCs make a satisfying splat noise if they fall.

Side note: The stairs to the surface around the shaft seem to be about 60-odd flights [1], so presuming a 10' rise per flight, the ceiling isn't 600 feet. But that's still an awe-inspiring mental picture.

[1] Someone with a move of 7 and a high enough HT or Running that they didn't have to make significant rest stops can run the steps up the shaft in about 7m 9s. The winner of the Empire State Building stair climb did 86 flights of stairs in 10m 16s, let's say he's our Move 7/healthy runner. That suggests the steps are ~60.5 flights of stairs. Unless I have my math wrong, which I could. It might be 390' instead of 600 ft.

[Joe]

Originally Posted by Bruno

I'm noodling around with making a model of caverntown in Blender

Do it! That would be fun to see. I must admit that, despite all the reasons not to make a map of the place (reasons which Kromm goes through in the supplement itself), I'd still find a map more helpful than harmful.

[Dalin]

Originally Posted by Bruno

Assuming the rough default of a 1.25mi (2km) on a side space for Caverntown, and not fiddling with the ratio of height to x-y dimensions, how high is the dome ceiling of the cavern?

The cavern is described as a "vast hemisphere" and we know that it is circular because the walls are 500 yards from each of the four gates to the square city. Since we know that the city is 2200 yards across (1.25 miles), the diameter of the cavern is 3200 yards. If the cavern is a true hemisphere, then the ceiling should rise half that height from the center of town (the shaft). So that's 1600 yards, 4800 feet, or about 0.9 miles. Plenty of room for dogfights and satisfying splats.

There is one wrinkle in the description: it says that the floor "meets the dome's descending sides at an acute angle." This suggests that the cavern might be somewhat less than a perfect hemisphere (with the floor above the "equator" of the theoretical sphere). So one could take 1600 yards as the maximum ceiling height of the default cavern, but you could reduce it if it seems too spacious.

[Dalin]

I don't have nearly the math chops of some of the folks around here, but a hemisphere is at my level of napkin trig. (Still, I hope someone checks my work!)

Based on the hemisphere radius of 1600 yards, I calculated the ceiling height at the corner towers (50 yards from the cavern edge, 1550 yards from the center) to be roughly 400 yards or 1200 feet. So no problem having tall towers. Above the gates at the center of each wall (500 yards from the cavern edge, 1100 yards from the center), the height would be 1162 yards / 3500 feet.

[Kromm]

The default city plan is a perfect square 1.25 miles on a side (Where's the Map?, p. 6). That's an even 2,200 yards across. The gates in the center of each side of the Barricade (p. 7) are described as being "about 500 yards from the cavern walls." That gives the Great Cavern a diameter of ~3,200 yards.

(You could also look at the diagonal: √(1.25² + 1.25²) = 1.76 miles = 3,098 yards. The four corners are described as being "a mere 50 yards from it." That gives a cavern diameter of 3,198 yards . . . which is still ~3,200 yards at the assumed precision.)

If the Great Cavern were a true hemisphere with the city sitting on a plane slicing a sphere in two, that would make the ceiling ~1,600 yards tall (i.e., equal to the cavern's radius). However, "the cavern floor [...] meets the dome's descending sides at an acute angle" (The Great Cavern, p. 6), so it isn't that spacious. That is, the cavern floor isn't a plane that slices a sphere in half. While the ceiling and walls describe "the surface of a vast hemisphere," it's never stated what percentage of that hemisphere is open space.

Looking at the stairs described under The Shaft (p. 6), I assumed the walking speed of a fit person marching with purpose – i.e., a guard or adventurer – is Move 2 (2 yards/second, or ~4 miles/hour), which tends to be what GURPS assumes elsewhere is a brisk walk (see, for instance, Invisibility Art, p. B202). I further used the standard approximation that stairs cost +1 movement point per hex (p. B387), with the idea that as in ranged combat, the implied slope is around 45°. Thus, Move 2 behaves like Move 1 upward and Move 1 forward, and so the stated 25-minute journey is a climb of 25 minutes × 60 seconds/minute × 1 yard/second = 1,500 yards, not 1,600 yards.

Moreover, some of that distance is passage through the Great Cavern's stone ceiling. That isn't thin, because it's mountainside that supports forts and so on despite being riddled with tunnels and caverns. Still, we don't want that acute angle to be too acute or you'll be bumping your head! So the part of the stairs that disappears up into a little hole in the ceiling spans at least a few stories. I'd put the zenith of the cavern ceiling – above Town Square – at between 1,400 and 1,500 yards.

As Where's the Map? notes, "The GM may adjust the ratio of cavern diameter to height, too; the greater this is, the more oppressive the space will feel." Thus, it's up to the GM to decide whether caverns nearly a mile deep make sense in the setting. That said, note that this isn't the same as "almost a mile below sea level" – Fort Caverntown is up a mountain and well above sea level! Rather, it's "almost a mile below the surface."

For those preoccupied with atmospheric effects: At GURPS' resolution, you would run into these only if the fort were at least 2,000 yards above sea level (p. B429) or the cavern floor were at least a mile below sea level (GURPS Underground Adventures, p. 13). So if you split the difference and have the fort be 800 yards above sea level (comparable to São Paulo) and the cavern floor be 800 yards below sea level, you could accommodate the above assumptions without worrying excessively about pressure at the resolution likely to matter for hack 'n' slash fantasy.

In short, there's really nothing preventing impressive magically built skyscrapers in Caverntown. As Buildings (p. 7) notes, "most structures rise at least two stories above the street. Many are higher, their height reflecting the owners' wealth and status." The only restriction, if you want to call it that, is that the Wizards' Guild tower remain the "tallest structure after The Shaft and the Eight Titans" (p. 21). Likewise, aerial adventures above the city are entirely plausible, and there's even a flying species native to the Great Cavern (p. 12).

[Blind Mapmaker]

Now that's what I call support for all the niche interests.

For me it's more important to know when it is permitted to use the stairs? Because the staircase is described as auxiliary and off-limits to visitors in the Fort Caverntown section. Does that mean native inhabitants of Caverntown may use it or is it open only to guards and in emergencies? And what kind of emergencies would that be? If the town is attacked, you probably don't want citizens gallivanting on the perfect sniper platform. It certainly would make for an epicspot to watch a zombie apocalpyse unfold and engulf the town.

[Refplace]

My review is up.
https://refplace.blogspot.com/2018/0...averntown.html

[SolemnGolem]

Bought and downloaded - will post thoughts later.

I just wanted to echo the others in wishing Dr. Punch a speedy resolution to any of his difficulties, and thanks for continuing to feed our ravening appetites for all things GURPS.

Also, it's pretty amazing that Denis Loubet is illustrating GURPS products. I first saw his illustrations in the player's guide to the Ultima IV video game...

[Bruno]

Originally Posted by Kromm

I'd put the zenith of the cavern ceiling – above Town Square – at between 1,400 and 1,500 yards.

That actually means the walls at the edge of the cavern aren't meeting the floor at a terribly acute angle, all things considered. Taking an aggressive height of 1400 yards, the slope at the edge at PC height is roughly 1' horizontal to 6' vertical.

It also means the Giants being 10' wide look... spindly. Even upgrading them to 10 yards wide they look frail to my eyes. I know they're magically strong so there's no realism concerns, it's just... they visually get drowned out by the size of the cavern.

Fortunately 3D being the forgiving medium that it is, I should be able to make two versions of the model, one with 10' pillars and one with stout ones with stouter ones and a note with how wide they end up.

I'm debating between making the Barricade walls a magically fortified 100' tall or a "tall but historical" 40'. After a certain point (you want it for fending off ground bound and giant critters), the wall feels a little futile because of the problem of flight.

[Kromm]

Originally Posted by Bruno

That actually means the walls at the edge of the cavern aren't meeting the floor at a terribly acute angle

Nope. But knowing how gamers like to analyze things, I wasn't going to say anything but "acute angle." ;)

Originally Posted by Bruno

It also means the Giants being 10' wide look... spindly.

That's intended! The idea is that the Eight Titans are totally fantastic and beyond any sensible explanation. That's why I draw attention to the fact that "Where The Shaft fills most of a block, the Eight Titans are only 10' thick," and go on to note that "The pillars themselves are effectively indestructible." It may help to think of them as Essential Stone, which means they have 3× the HP . . . which in turn means 27 times the mass . . . which for fixed length means 27 times the cross-sectional area and thus 5.2 times the diameter. So they'd need to be 17+ yards thick if they were natural.

Originally Posted by Bruno

they visually get drowned out by the size of the cavern.

Also intended. The idea is that you can see most everything from The Shaft. I didn't want huge blind spots, geometrically or aesthetically (i.e., cluttering up the vista).

Originally Posted by Bruno

I'm debating between making the Barricade walls a magically fortified 100' tall or a "tall but historical" 40'. After a certain point (you want it for fending off ground bound and giant critters), the wall feels a little futile because of the problem of flight.

It's "one Climbing roll tall," so canonically no more than 60' tall (per p. B349, a 60' vertical stone wall would take even a fast climber 300 seconds, or five minutes, and thus demand a second Climbing roll). The biggest standard ground-pounding monsters – and thus the ones people who build walls would know and care about – have SM +5 and therefore a maximum extent of around 45' all stretched out. A wall significantly taller than this is unlikely to be worth the trouble, especially as flight really does make that pointless (which probably helps to explain why the Mayor is obsessed with bigger and better crossbows and scorpions). So I'd peg height at between 45' and 60'.