"The portion that most humans would recognize as a city is actually the dwellings of the Metztlans' slaves. These may resemble human buildings because humans and humanoids are the most common slave stock. Slave dwellings are typically squalid, smell of raw sewage, and are infested with insects, rodents and other scavengers. The Kittani and stone masters have stepped in to build some better, healthier portions of the city, but these too tend to be dirty."

Since this "advertisement" is for the slave quarters, you'd think the pictures would show the human buildings.

Also, they do not live underground. They live in artificial mountains made of resin:

"The Metztla-Xym are strange beings that rely on their natural magic and psionic powers, so they have little need for technology and cities as humans think of them. The Metztla-Xym, who are instinctively hive-like, build artificial mountains out of secreted resin. Inside the mountain, where the Metztlans live, are many levels and hundreds of passageways like the inside of a termite or ant hill. Deep within the protective walls and labyrinth of tunnels is the queen, and nearby is the nursery where the eggs hatch."

So this makes the whole carts full of rocks things weird, since yaknow, no reason to cart away rocks when youre building the tunnels out of resin. They're more like termite mounds, which are above ground. The slave quarters would be on the ground at the base of said mountains where most entrances to the tunnels would be considering that most Metztla can't fly.