The Citadel of Silver Light is the homeland or headquarters for the Geisterdamen; all the ones that we have met to date in the story were to Europa by The Other to hunt for the Holy Child, who was evidently born there and then from the Citadel by person or persons unknown. The Geisters made the trip through a Queen's Mirror, which has since been or disabled in some fashion by the rogue Geister Loremistress Milvistle.
Beyond that, almost nothing is currently known about the Citadel by either the readers of Girl Genius or people in Europa. although some folks in the latter set have at least heard of the place: when Zolaa copy of Lucrezia's memories to rifle through, she comments: "Ooooh. And that's why no one's been able to find the Citadel of Silver Light. Amazing! it explains so much!"
In Agatha H and the Clockwork Princess, Vrin spins a somewhat longer tale to Agatha about life in the City of Silver Light and the taking of the Holy Child, but still doesn't give much in the way of an actual description.