A deranged magic-user is controlling swamp beasties and sending them into a city's sewer system. Why? Well... uhm... ya see... uh... *ahem*
 

Yes.... that's supposed to be a woman in the middle.

What is a town of this size doing on the edge of a swamp?  What do the people do for a living?

Oooooh..... Player Hand-Out!  I must have been feeling especially clever.  Anyways.... this is what the sewer system is SUPPOSED to look like, according to the city archives.  Don't ask me why the city well opens into the sewer system.  I really have no idea.

Here's what the sewer system ACTUALLY looks like.  I have yet to figure out how the city managed to remain standing rather than collapsing into a giant sinkhole.

The seven story tower in the middle of the swamp where the deranged magic-user dwells.  How he keeps this thing from sinking (or how he even managed built it out there) is a complete mystery.  Another mystery is why nobody from the city seems to know about a 70-foot tall tower out in the swamp.