That place is surprisingly tough, for a dungeon near the start of the game. Like Bones says, make sure that all your party has bows together with a decent amount of bow skill, otherwise the lobber pods will eat you alive.Turning to the dungeon itself, the ladder back up from the "cobweb" level (I call it the Beet Hoven Catacomb, or BHC for short) to the top-level sewers is not obvious because it's inside a well that dips into an underground stream. The stream is your marker. You have to dive into the water and come up inside the well, only then will you see the ladder back up. Getting back up to the BHC from the lower sewers is easy because there is only one way up.
Incidentally, in the BHC, you will find some rooms with sarcophagi. Some of these contain mummies, which will attack you if you disturb them - all you have to do is go too near its sarcophagus. If you can kill them quickly enough, they will leave treasure. If they die slowly, they will take their treasure with them.
