So that city campaign I'm running is going to need to drastically change. D&D just wasn't designed for something like that; It's incredibly hard to come up reasonable events and tasks that could challenge a party of even lvl 3 PCs in an environment like that.
When I say "reasonable events", I'm talking about things that wouldn't cause massive damage to the city and lead to major retribution against those involved.
So it's time to start thinking about unreasonable events.
So I'm thinking about a natural disaster.
First, the city sits on a very large island; The city proper takes up a big chunk on the coast, but the rest of the island consists of either mountains or agriculture. The mountains form a large coastal C shape around the farmland; It's not unreasonable that they might be the product of two continental plates colliding. So there might be a massive earthquake. But I like the next approach better.
The mountains form a basis for a kind of continental divide. Since they form most of a ring around the island, virtually all rain that falls on the island flows a massive network of rivers and streams, much of which ultimately flows under the city as its self-flushing sewage system. The farmland itself is largely flat. Sure, they get occasional floods, but the construction out there includes shelter, and minor structures are easily rebuilt. Water that can't fit through the city's sewage tunnel network normally would overflow into a side channel, and out into the ocean. Combine a major hurricane with a little villainy (Such as a wall of force blocking the overflow), and you've washed away 90% of the city, and left much of the rest covered in sewage.
I think I like the hurricane approach better. The event itself will be interesting to play out as the party scrambles to find shelter. (Best bet, I think, would be to get on the city walls.) Following the hurricane itself, the city will be a collection of laes for several days as all of the different districts drain. Central city authority will be devastated, food will be scarce, and most wooden structures will be destroyed or severely damaged.
With food and clean water scarce, those who don't form a massive exodus into the farmlands will fight for what resources remain in the city. New power structures will form and conflict, and power structures the player characters are already familiar with will be caught up in the struggle.
It's going to be interesting. I'm not sure if next session will have the natural disaster take place, or if some foreshadowing is required. (The closest thing I can think of to an early warning system is a bunch of clerics casting Divination, though there may be gnome weatherfolk who watch things like atmospheric pressure.)
I'm thinking of a category 4 hurricane, with a number of tornados thrown in for good measure. Thanks to the beauty of plot devices, the tornados won't hit the PCs, but it's going to be tough.
Hm. Considering the city walls will be used for shelter, let's put some thought into that. Let's say they're stone, hollow, and two levels high on the inside, and a third level on top. Figure they're designed to counter internal conflicts like riots and civil war, not attacks from outside the city. As such, there's likely to be two hollow wall structures for each district border, with a gap in between for buffer and travel.
In peace times, the gap between these two hollow structures is covered and contained, and serves as a sort of thoroughfare and bazaar. Effective shelter from most wind and rain, but not effective against tornados (Rip the roof off...) and flooding. The holow stone structures will be stronger shelter; Their lower level will be immune to wind and tornados, but may have flooding problems. The upper level will be immune to the first stage of flooding, but won't provide from drowning for small creatures. (And the party consists of halflings...).
I think the whole storm is going to last a week..
Day 1: clouds, light rain
Day 2: heavy Rain
Day 3: Heavy rain, thunderstorms, a tornado hits somewhere in the city Winds destroy some wooden structures.
Day 4: Severe thunderstorm, %10 chance of a tornado in their part of town every hour. Eye passes through a different part of town in the night. Winds destroy most wooden structures.
Day 5: Severe thunderstorm, %10 chance of a tornado in their part of town every hour. First level flood. Some wooden structures remain, but are unsound.
Day 6: Heavy rain, thunderstorms, another tornado hits somwhere in the city. (25% chance it's in their area.Second level flood. Stone walls perpendicular to flow of waters at risk. Only wooden structure in calm flood areas remain.
Day 7: light rain, clouds, and, finally, sunlight. Flood level maintained at second level. Gaping holes in stone walls allow quick flow of flood waters through the city.
Flooding will ease to first level on day eight, and will finally seep to below street level (i.e. through the sewers) on day nine.
It's going to be a rough, challenging session. :-)