By day, the streets of the Wicked City are more sad than dangerous. The statues of the king stand on every corner, and anyone who says or does anything seditious where the statues can see or hear them will promptly be disappeared by the secret police, but aside from this the tyranny of the Wicked King rests a little more lightly, here; patrols by the King's Men are relatively infrequent, and the districts are largely left to run themselves. The people grow vegetables in what were once parks and gardens, and scavenge amidst the rubble for materials with which to keep their own homes in repair. By night, they take on another aspect; at dusk the people bar their doors and shutters, and refuse to open them for anyone they do not know. Navigating the rubble-choked streets becomes weirdly difficult; roads seem to twist and turn like serpents, buildings rearrange themselves at random, and paths unexpectedly turn into dead ends while no-one is looking. Through this bizarre, shifting nocturnal cityscape roam the other inhabitants of the districts: the crazed hermits, the criminal gangs, the malfunctioning clockwork robots, the junkies who've taken so many weird serpent-man drug cocktails that they're barely even human any more. Enormous vermin, too; giant rats, giant spiders, giant cockroaches, giant centipedes... no-one ever sees them during the day, but by night, there they are, digging through the rubble and scurrying through the abandoned courtyards. Most are the size of dogs, but some are bigger. Some are much bigger.
Smart PCs will make as many friends and allies as possible amongst the people of the streets. By day, being friends with a few families who live out in an otherwise-deserted district might not seem like much; but by night, when that same district turns into a nightmare of looping streets and shuffling buildings and drug-addled crazies and rats the size of ponies, having someone who's willing to open their doors to you can mean the difference between life and death.
(Of course, the really smart thing to do would be to avoid going out after dark entirely. But since when have PCs ever been able to manage that?)