There were some naturally-occurring caves, but also the following:

• Some eldritch horrors sleeping in the deepest caverns beneath the city who act as a “lodestone” for evil creatures, compelling them to come here.
• An ancient cleric once attempted to build a massive vault here as a safe depository for evil artifacts. He became corrupted by them, and turned into the first Evil Overlord.
• Much later on, a second Evil Overlord appeared who wanted the secrets of the first Evil Overlord. He expanded the caves and turned them into his military headquarters for his conquering horde of monsters.
• There was also a major dwarf hold here at some point before they abandoned it.
• There is an entire drow city deep beneath the surface.
• Assorted other groups of monsters keep showing up and making themselves at home.
• Oh, and the current incarnation of the city has been built on top of this, which includes a massive sewer system that is gradually breaking down from lack of maintenance.

The whole dungeon system has developed over the course of more than 8,000 years. The general inclination of the citizens above is to wall off any entrances they find, and otherwise let adventurers deal with it. (This is another unique aspect about this city - it’s the only one on the continent which has a professional class of adventurers.)

As for the Thieves’ Guild, they tend to grease a lot of palms in the city watch, so they don’t have to worry much from law enforcement unless they do something too blatant. However, there is a new criminal organization which is giving them a lot of trouble, and there are a lot of adventurers around, so they do have reasons to take their security seriously.

The guild have mages in their employ who could help with magical security, but none of them is present in this safe house.

The main headquarters are elsewhere, but the guild maintains lesser safehouses in most districts of the city.

But you are right, I haven’t put much thought into whether this is a hub for activities or a place to lay low. I’d say it’s primarily the former.

The city of Ptolus has a long and convoluted backstory, and is known across much of the continent as “that weird city with all the dungeons and adventurers”. Suffice to say, there are reasons why the region attracts all kinds of supernatural creatures, and there were many, many different groups of people and creatures who expanded these dungeons.

If you like the whole concept of “dungeon delving mixed with city adventures”, I can highly recommend Ptolus. Few do it better, or more exhaustively.

The closest I can think of is the “Earn Income” downtime activity, but that’s not exactly the same as what you pay NPCs.

Here is a neat bit of ancient technology: A qanat.

A common problem in arid regions is how to get enough water for your irrigation needs. Digging wells is one possibility, of course, but the water table might be far beneath the surface.

However, the neat thing about the water table is that it runs parallel to the surface - so if the terrain rises up and forms a mountain, the water table will rise up beneath the mountain as well. Thus, you can tap the water within the mountain simply by digging a tunnel into the flank of the mountain.

Which isn’t exactly a trivial undertaking, of course. Still, some qanats in the Middle East have been in use for several thousands of years.

And for #TTRPG , such qanats represent a good entrance to the “Underdark”, or whatever the local “Realms Below” are called - or vice versa, and monsters might emerge from them. And what happens if a qanat suddenly ceases to bring water? Naturally, some daring adventurers have to go in there and solve the problem, or else an entire community might starve!

Pathfinder 1E or 2E?

I don’t have much practical experience with the latter, but it did move away from the notion that NPCs must be created with the same system as PCs.

Yeah, GURPS character creation can take quite a lot of time, but once the character is done it flows very smoothly, so that doesn’t bother me. But GMs need to create NPCs all the time, and the speed of character creation is very, very important.

I particularly like the monastery - we need more places like this as adventure locations!

Another candidate: The Centralia Mine Fire, an underground coal seam fire that has been going for sixty years, and which could continue to burn for 250 further years!

I wonder how dwarves or other subterranean civilizations would deal with something like this?

Another one: The “Bergpark Wilhelmshöhe” in Kassel, Germany.

This is a masterpiece of Baroque landscape architecture, and the Hessian landgrave at the time was only able to afford it because his father sold of Hessian subjects to the British so that they could put a stop to those pesky rebels in the North American colonies. The park is built across a hill slope (and covers an elevation change of more than 250 meters). Its highlights are the “Wasserspiele” (“Water Plays”). On every Sunday and Wednesday during the warmer seasons, water is released from a vast cistern at the top, located beneath a giant bronze statue of Hercules. Over the course of 75 minutes, this water flows down a series of artificial waterfalls and channels until it powers a giant fountain close to the bottom of the park.

Beyond that, the park has all sorts of other attractions - a fake ruined castle, a fake ruined Roman aqueduct, and a series of miniature temples to assorted Roman gods. This park makes a perfect setting for all sorts of cinematic adventures and/or occult weirdness!

I took it from Wikipedia, which says that it is from the Monteverde Cloud Forest Reserve in Costa Rica.

Truly, a scenario to warm the shriveled heart of any veteran GM.

From what I gather, there were actually fewer accidents under this system than with the ladders.

There’s even one still in operation, at Sankt Andreasberg where it’s used for maintenance at the local hydroelectric power plant installed in the former mine shaft.

I wish that someone had warned me that one of the most important aspects for picking a system to run is how difficult or complex it is to create NPCs or monsters.

I ran D&D 3.X for a time, which… wasn’t great for that.

Then I ran Exalted 1E and 2E, which were worse.

These days I mostly run D&D 5E, which is (a) vastly simpler when it comes to NPC prep, and (b) has so many stat blocks in both official and unofficial sources that I rarely need to come up with something custom.

A honorable mention goes out to GURPS, which is actually pretty easy to run for once you know what you are doing - first you need to keep in mind that “character points” are mostly for player characters, and can be ignored for NPCs for the most part. Then you also need to keep in mind not do overdo it with defensive stats, or else combat will get bogged down and boring.

The author isn’t wrong, and such issues are worth thinking about when either running campaigns or doing #ttrpg worldbuilding. At the very least, dungeon inhabitants tend to be people or creature who live in this marginal environment because they were pushed away from more fertile regions (such as the fertile surface lands typically inhabited by player character ancestries).

Even if you do use some of this default structure, it’s worth introducing some scenes and elements that could make the PCs (and the players) think: “Hold on, are we actually the good guys here?”

I’ve covered a few tales of the Holle/Hulda/Perchta myth-complex in my translations.

(I see these three as fundamentally the same entity, even if there are major regional variations.)

I’ve covered a few of these tales in my translations.

I suspect that there is quite a lot of overlap with kobolds and household spirits in German folklore.

I suspect that there are quite a few of these in German folklore. I have collected more than a hundred German folk tales of saints and holy people over the last few years, but I haven’t cross-referenced these with the list of canonical saints yet.

I do hope they use the ORC license for this, like they do for their other games.