A special level (possibly in a quest, Rogue-locate is a good candidate) level that contains a pond outside some building or city wall, with one iron bars square on the other side of the wall, inaccessible except by going over the water. Behind it is a (maybe randomly generated?) network of corridors inside solid rock representing a sewer. Either there’s something in here required to complete the level, or there’s an alternate path to complete the level, or there’s some treasure.