The cartographer drew this handy sketch to let us know that the northern portion of room #19C is accessed via a corridor that goes underneath room 18B. This is good to know cuz that's not clear from the map; one might otherwise conclude that there is no way to get there. I draw these sort of thumbnail sketches of the more convoluted portions of my dungeons all the time; it's nice to see one in an old time TSR adventure as well.
That said, what's the significance of the space? Is it essential that it goes under 18B above? More important, is it interesting enough to merit a special sketch? Well, it's the machine room for the works that open and close the slave pits in room 18--the actual levers that control the works are upstairs in 18b--and it's occupied by an aspis drone and a few slaves whose job it is to oil the cogs and such. It's nice that they acknowledge this sort of infrastructure, but is it worthy of the sketch? You be the judge.
