One of the most exciting parts of building the layers of a novel is playing the characters off one another. I love it when I know something that my heroes - or villains - don't know. It makes me feel like I'm in on a secret, and builds dramatic tension.
In just a few simple words - "over in Ruchus" - the reader has suddenly been given a huge chunk of information. The CItadel isn't the entire Mechalarum universe. Though it has ostensibly been cut off from the rest of the world, at least some of its inhabitants are not quite so imprisoned.
Things were different, in the early drafts. The Citadel was a last bastion of humanity, instead of a base from which to launch a secret attack. Kiellen and Gage might not know much about the world at large, but Kiellen at least knows she's headed there.