This is yet another I-commented-on-this-somewhere-and-I'm-not-done-talking-about-it posts. Over at Hill Cantons there is a good post about running ruined city-based adventures, and the comments section has some good information in it. Below, I've reproduced, edited, and expanded on my comments.
I haven't run all that many ruined city adventures. Not in GURPS, and not in D&D, and not in fantasy gaming. But I did run an adventure or two - frankly, most of one campaign - in a ruined city in Gamma World. This one specifically - 2nd edition - since it came with a ruined city based adventure. The "Gamma World Adventure Booklet" was a guide to setting up adventures, a wilderness adventure location (the lands around ruined Pitz Burke), a starter adventure (a reason to go to Pitz Burke), and acted as a bit of a guide to running adventures in a ruined city. So I used that when we moved up from 1st edition Gamma World to 2nd edition.
Based on that experience, these are the issues I noticed:
- A ruined city is not a dungeon, it's a wilderness with channeling terrain and lots and lots of easy ambush points. Think jungle not plains. If you turn it into a dungeon (narrowly channeled paths, roof overhead, etc.) expect the players to treat it like one. If you greatly limit their movements, punish them for flying above the ruins or leaving them and circling back in, etc. - you're just making it a dungeon. B4 The Lost City and the Conan story Red Nails are both nominally city-based, but act a lot more like dungeons. The first is a dungeon, with some talk and pictures of a city elsewhere that you have to stat up and map yourself. The second is a single big city-sized building, but it's chock full of secret doors, fights in hallways, and people stuck inside. And a roof overhead - it's a very dungeon-y city. This can be fun but it's not necessarily the best way to handle a city.
- In a ruined city it's tough to give a complete impression of what they can see. Visuals help a lot more than in dungeons. You need some way to show them the size of the city, give some sense of scale and what's there to go to - and what's between here and there.
- You really need wandering monsters and pre-set encounters in a mix, or there isn't anything to do. I1 Dwellers of the Forbidden City does a great job of this. Again, think wilderness. Some dungeon-like locations are great, so are communities, nests, and other encounters in the ruins. Even a dungeon in it (well, under it) is a good idea. But much of the travel will be above ground and meandering, so you need a way to basically stock on the fly. This is a good time to roll % in lair, too - was it wandering by or does it live where you ran into it?
- you need to have a good plan for dealing with improvisation and searches for unexpected materials or locations. Random tables for monsters are one thing, random tables for buildings might help, too - does the sort of building they're looking for exist? Where is it in relation to them? (the dice drop method in Vornheim works here, just put ruined stuff between here and there instead of "live" city.) Another option is a quickie ruin generator - is this building they're heading to intact, semi-intact, ruined? Is it safe or unsafe? Inhabited or not? Etc.
- Part of the charm of using a big ruined city is that a) you can go anywhere. It's like being in a real city, only since it's largely uninhabited you can go around and poke into all those buildings and look behind doors and look down the manholes. That's part of the fun. And b) you get the feeling that anything could be in there - you the player, and you the GM. With a big enough ruins, and wide enough random tables, it could really be there. You could equally not know all the nooks and crannies of your city. So it'll feel like there is so much you couldn't clear the place, find everything, or really know everything. It'll make perfect sense when you run into Generic Old Coot living in the city and he says he's been there 50 years and doesn't know it all quite yet. That's part of the coolness of ruins.
Remember, wilderness with lots of easy ambush points and places to look at. You'll need to know the sighting rules and whose keeping an eye out for what. You'll need to know weapon ranges and who'll engage at what range. You need to know how damn far noise carries above ground if the players make a lot of noise, like my neighbors did last night. It's a wilderness with cover, more than a dungeon or a city.
True story - I almost started my DF game using I1 Dwellers of the Forbidden City. The main reason I didn't was that I wanted people to be able to come and go, and that setting isn't conducive to "and we go back to town and replenish our supplies between sessions." Pitz Burke would work okay for that, if I run a post-apocalypse game again . . .
Some good resources to look at:
Besides what I mentioned above:
"Ruins: Rotted and risky, but rewarding" by Arn Ashleigh Parker, Dragon #54 / Best of Dragon Vol. V
AD&D Wilderness Survival Guide (the sections on spotting/visibility, especially)