Very nice. I'll need to get this printed out and put with my Blackmoor books (or get it printed out big and make it into a poster).
Your style reminds me a bit of the 2.5D dungeon map tiles created by theDMG and DM's Craft. I wonder if it would actually be possible to build your sewer system, using their methods.
One thing I am wondering (if you are looking for constructive criticism) is if one feed in and one feed out is enough for a big city.
Havard posted your map with a map of the surface of the city of Jackport superimposed on it and it looks like you need a sewer inlet going north and maybe a second inlet going west to provide enough drainage for the city. I wonder if your pipe leaving the west of the map could have a dogleg with a second branch going to the Old Market. I wonder if your original "straight stairway into the sewer" could be an other sewer going west towards the House of Father Aeris, with the new improved ladder coming down from an inspection manhole in a long sewer.