The Citadel is a massive and ancient castle erected on Heliopolis by the original Alliance of Great Races to act as a "United Nations of the stars," where interstellar diplomacy could take place and sensitive talks could be held. Following the collapse of the organization, however, the structure fell into disrepair, its stargate's dialing device rendered inoperable and its rocky base eroded into the ocean. It was in danger of sliding into the sea itself by the time SG-1 visited the place during the first year of the Stargate Program, and the Citadel was largely ignored for the next decade, leaving its survival up to fate.
When the Nox contacted various modern species to put together a new Alliance, they recognized not only the need for a meeting place, but the symbolism of the old Citadel still on Heliopolis. Utilizing architectural styles from powerful races both old and new, they rebuilt the castle into a sleek and magnificent complex again worthy of its purpose. Demonstrating their own technological prowess, they even utilized small-scale terraformation to mold the terrain into something more suitable for a modern metropolis, laying the groundwork for the city that would eventually come to life around the Citadel itself.
Now containing the offices of the Interstellar Alliance government and the Director of its intelligence agency, the Citadel represents the might and authority of the entire galaxy. As it has sporadically for millennia, the complex has again taken its place as a beacon of power and solidarity to all those who wish for peace.