The Sewers are accessible from Floors 3-5. The sewers lead to the River. The Sewers have a 50% chance of appearing each game.
In this branch you will find rats, bats, spiders, snakes, and sludges.
Lurking in a dark room is the Rat King. This is a fairly easy optional boss (it drops no ring), but it is very fast. If you land a hit on the Rat King, it spawns a Rat Soldier. The main danger when fighting the Rat King is that its speed is likely to cause the Bleeding status. It can be much easier to fight this boss from range or as a vampire.