As you know, each Canto of Dante's Inferno is set in a different "hellish" environment. Canto 18 is no different. In fact, Canto 18 is set in a special part of hell called "Malbowges" otherwise known as "Evil Pockets."
Malbowges is specifically designed to show the moral breakdown of the city and continually makes a reference to "in the gutter" as a result. In the center is a well. There are the gravest sinners submerged in the well (full of "filth") such as those who seduce others and those who "pander" which, in modern terms, means to act as a pimp gaining clients for whores. The well is, of course, apart from "the gutter" that contains flatterers and, specifically, those who flatter and tell lies through words. In "the trench" or "the gutter," the narrator finds Alessio Interminei (a famous Italian flatterer) and also Thais (who is also a prostitute, but has not been submerged into hell for her prostitution, but for her use of words).
Another interesting aspect of Malbowges is the directions of the eyes of the souls. Half of them have their eyes one direction: on the castle. The other half of them have their eyes in the other direction: on the mountain. The narrator figures out that a guy named Venedico Caccianemico Ia man who sold his own sister into sexual slavery) is coming from one direction and Jason of the Argonauts (a famous seducer) coming from the other direction.
Just remember Dante's very famous quote that proves Malbowges is not the in the furthest depths of hell:
The darkest places in hell are reserved for those who maintain their neutrality in times of moral crisis.