if you take the road leading north after the castle, you should come to the market place, there's the entrance in the middle of this area.
Or, go to Lain/poor quarters after you're a member of the thieves guild in Verdistis = hatch in his house, so the way underground is shorter.
Caution: Sewers are full of lizards.
Kiya
Rivertown map by Dragon, click hereit's a very large and detailed map, so it takes a bit to load <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Nr 5 on the map is the sewer entrance.
East house is the hatch to the sewers.