GURPS Dungeon Fantasy Setting: Caverntown

[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).