Castalia was shaped like a peanut. As it was occupied by Formics, they had created a resin shell around it that contained the atmosphere necessary for their purposes.
Castalia was determined to be an asteroid with a higher threat level than the others occupied by Formics, and so a International Fleet ship was sent to meet it. When it arrived, a breach squad consisting of Mazer Rackham, Kaufman, Rimas, and Bingwen was formed to infiltrate the shell around the asteroid and see what was going on inside.
Two days later, the breach team was ready to go. They planned to kill the crew of the Formic miniship, collect the bioengineered "bugs", and then send in Bingwen once the threat was eliminated. Mazer and the men investigated the surface, but left as soon as they realized they could not fit inside the tunnels. Bingwen instead investigated further, where he discovered a wide cavern in the center of the asteroid where he came to the understanding that the Formic asteroids were ship factories, not missiles. He then saw the scene of a Hive Queen being birthed, who commanded the workers to swarm him after hatching from her cocoon. Bingwen pulled out an ignitor and blew the atmosphere, destroying the entire asteroid. However, the Formic ship survived, but was inaccessible due to it being made out of Hulmat. The International Fleet planned to send a Zipship with Juke Limited's nanobots that could destroy hulmat in order to find out what was inside the ship.