After I have chosen one of the two route, I came upon a place full of fire. However, I cant get through the door because the gate closed when I go near it. There are two levers but turning those lever can't seem to work. Can anyone help me?

The two levers will open the door, but DON'T cross the fire in the middle, coz this will close the door.

Yamatetsu

I am now struck in the crypt after getting the signet from Mavin. Does anyone know the way out of the water? I have been going round and round but dont seem to have a way out.

use the lever where you found the gold key, its on the wall under water.

anders