"The Leeran city rose from the seabed like a wondrous tower, perhaps half a thousand feet wide at its base, dwindling to a few dozen feet at the very top. The top pressed right against the sparkling water ceiling, up to the border between sea and sky. At the very top, huge fans sucked in air and blew out exhaust from the entire city. The city itself violated every logical law, at least as far as Andalites or humans were concerned. [...] The predominant color was pink. But there was blue and green and purple as well, in vast swatches of seemingly random color. Openings were everywhere. Leerans drifted in and out and around and through, a hundred feet up, twenty feet below us, everywhere. And like some slow-motion tornado, the long, electric-blue worms swam around and around the City of Worms. They formed an eerie halo. Even as strangers, we could tell the city was tense. There were weapons poking from many of the windows."
Aximili-Esgarrouth-Isthill describing the City of Worms[src]

The City of Worms is a massive Leeran city found under the sea of planet Leera. The city resembles a massive, multi-colored, shell full of holes that Leerans could enter into. During the Yeerk invasion of Leera, the city was fortified with loads of Leeran beam weapons, and was used as the headquarter for the Andalite forces during the invasion of Leera.