Greede's Underbelly is a massive maze that is used to keep monsters away from the Demithor. It is the only passage to the Demithor's Head, and is overrun by monsters. Count Drisdall and Amir took Caesar here as a child.
Here is a tip: Everything is not what it seems. If a door or drain dosen't open, there is most likely a switch nearby that opens it. Make sure you have full Ac throughout this level because there will be ALOT of monsters.