the citadel

The Citadel is located in City 17, and resembles a massive, metallic skyscraper standing tens of thousands of feet into the air. It is a fortress that rises both high into the sky and extends a great distance underground as well. It is a triangular structure when viewed from above, and is crowned by a Dark Energy Reactor, with its associated portal acting as the primary means of communication with off-world Combine forces. The Citadel's massive interior level is capable of housing a massive army of Overwatch Soldiers, with the building itself being heavily mechanized, and entire sections of it can move to allow for mass deployment of airborne Combine units such as City Scanners, Hunter-Choppers and Combine Gunships. In the upper floors of the citadel, a large factory where Combine Gunships are repaired and augmented can be found in the Citadel, and a large flap on the Citadel's exterior lifts up to allow the synths to exit the Citadel to continue the war against The Resistance.