Was* the Combines main base of operation on Earth.
This ridiculously huge fortress used to house a stable portal that was used to pass between our world and the Unknown, now however, it is in a constant time loop.
When the dark fusion reactor of City 17's Citadel exploded, the blast subsequently destabilized the main Citadel reactor, and also forced a damper on the entire network of linked Citadel reactors.
This chain of events disabled all Combine teleportation and inter-dimensional communication systems on Earth.
Combine forces that remained within the damaged Citadel managed to disable the main reactor's containment system and attempted to accelerate the unstable reaction and send a signal back to the Combine home world- fortunately however, they were not successful.
Dr. Kleiner notes in one of his messages to the Resistance that the suspension of the City 17 Citadel has also caused the other Citadels across the planet to shut down.
Stuck in the events leading up to it's demise seemingly indefinitely (thanks to Dr. Kleiner and his random Dark Matter equations) it was locked into a destabilised time fracture that seems to be resetting preventing catastrophe.
Father Grigori has since taken residence in the tower.
Storming the Citadel
During the Weekend a large power surge can be detected and with the help of Dr. Kleiner, we are able to Enter the Citadel to try and clear out the anomalies inside.
A flurry of these surges can be detected with a 2 Hour gap between each of them. (See your local times on discord in #event-times!)
Technology can only hold the Rift open for about Fifteen(15) Minutes before closing and the fight begins.
Once inside, a large containment field can be found blocking the way.
Immediately after Fifteen(15) Minutes, the Portal closes causing the containment field to overload and shut down, allowing the Raid Group to enter and Ascend the Citadel.
The rest of the information seems to be redacted, i wonder what's in there?