The city of Stoneholt is generally considered one of the Wonders of the World, but it conceals a second wonder - a labyrinth of massive sewers and storm drains greater then anything man has built.
The system was well planned when it was built - it’s regular layout matching that of the streets overhead. It was built large enough to handle immense quantities of water, even diverting floodwaters from the occasional time that the Saar river overflows its banks.
The sewer system eventually delivers its waters to the sea, out of dozens of ports along the sea-sides. Originally, massive bronze grills prevented access via these points, but many of these have been salvaged for their metal. As a result, many denizens of the sea and other creatures have made their way into the sewer system.
Generally, exploring the sewers is not something taken upon lightly by people - these are still the domain of other creatures, not the least of which are the terribly fierce bands of feral caretakers. The Caretakers, or Shagru, are docile enough once broken, but the ones found underground are vicious and tenacious in the extreme, and much larger then normal. They are not alone down here. Massive 400lb rats, referred to as Sewer Bears, huge amphibious scorpions, and other massive vermin dwell in the dark. Rumors abound of even worse things, but those tales have not been substantiated.