Pathfinder 2E has a very similar power curve as D&D 5E. In fact, it might even be worse for Brancalonia as characters basically get their level as a flat bonus on everything.
I am personally not overly enarmored of OSR rule systems, but I'm a sucker for good worldbuilding and worldbuilding tools - I buy Sine Nomine Publishing stuff unseen, for instance. Maybe I should give this a closer look.
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 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.
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!
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.
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?
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!
Jürgen Hubert @ juergen_hubert @ttrpg.network Posts 50Comments 87Joined 1 yr. ago 
The situation in the AI industry being what it is, I would not rule it out!