Sewers

Beneath the opulent Upper City is a massive, sophisticated sewer system. The largest tunnels are wide as rivers and tower far above your head. However, unlike the heavily patrolled and safe streets of the Upper City, the sewers have been abandoned almost completely by the King's Government. The sewers are instead ruled ruthlessly by the criminal underworld. 

Much of the sewers are unexplored. The regions far from civilization have become overrun by mutated flora and fauna. If you survive the danger, there's said to be long lost treasures.

Life in the sewers will be rough. Down there, any rodent will slit your throat for the chance to get ahead. To survive, you'll have to be just as cutthroat. 

You could go to one of the many casinos, peruse a black market, or visit a bar. Or, you could venture out into the unexplored regions of the sewers. Be careful- one wrong turn and you're dead.