The PCs are likely are going to try to get into a safehouse of the local thieves' guild in the next two sessions. The setting is Ptolus, using the D&D 5E rules.
The safehouse in question is in a lower middle class region on the surface, but it has another entrance to the vast dungeon systems beneath the city. This, in itself, is not unusual - the bedrock on which the city stands is filled with tunnels and long-forgotten vaults. But while most house owners just try to brick those passages over, the thieves' guild actually uses these passages for their business dealings.
And I need some ideas on both what might be inside the premises, and what kinds of systems they use to secure the safehouse. The PCs will likely be coming from the "dungeon" side, and this is the kind of city where criminals expect inconvenient adventurers to pop up in all sorts of places.
I've already determined that the location includes:
- a vampire ally of the guild
- some cells for prisoners who get interrogated for information
- a bunch of drugs for resale
Any other ideas? The party consists of four 8th level characters, if this helps.
Consider what this place is to the thieves and how they use it.
Do they live here? If so there should be sleeping and dining areas. Do they frequently gather, perhaps to set out for work together, or do they tend to be dispersed, coming and going as jobs demand?
For the former, they most likely a barracks or common room and a dining hall. But if the latter, then something more like a series of tunnels and halls that occasionally have little apartments and safe zones. This is, to use your screenshot, basically the difference between the main guild area and the Ratways.
You say they expect adventurers? Do they set up a few rooms of traps and post a guard to catch them, or do they cleverly hide their entrances and then let monsters roam free outside safe areas as a deterrent, or do they welcome the (money and resources of) the adventurers and so the entrance is guarded but accessible, maybe almost like a lounge or casino?
There's typically a tradeoff between utility and security in any physical space. If it's easy to use the tunnels to get around the city quietly, then it's also easy to lose your enemies, to get lost, and to be suddenly infiltrated. On the other hand, if movement is difficult or dangerous or restricted, then the hideout is more secure from random incursion and discovery.