A small piece of plating barely visible on the left (southern) side of the Citadel also descends partly down the tower, scanners rapidly deploy from around that area on the tower, and the Citadel remains in this configuration for the remainder of the game. During this transformation a loud alarm resonates, echoing for several seconds each time it sounds, possibly emanating from the Citadel itself. Barney says that this is a rare occasion, stating 'I've never seen it lit up like that.' The inner plating retracts upwards, exposing a total of four sets of 'ribs', at the start of Route Kanal, where the reconfiguration has fully completed. The Citadel can be seen changing its configuration while Barney Calhoun gives Gordon a crowbar. Kleiner's teleporter, Breen puts the Citadel on alert, causing the structure to change to its alerted state.

However, shortly after Gordon Freeman was transported inside Doctor Breen's office in the Citadel during the malfunctioning of Dr. This configuration was signified by a single set of rib-like structures exposed within the right half of the Citadel, with an outer metal plating retracted slightly above the inner plating. The first of these is what was most likely the dormant configuration, which was probably maintained during the whole life in the city, when no serious threats were posed to the Combine (the structure also takes this shape during Half-Life 2: Episode One). The Citadel has two known 'states,' or configurations that the tower takes depending on specific situations. 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. The Citadel's massive interior level is capable of housing a massive army of Overwatch Solders, 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. Its associated portal was the primary means of communication with off-world Combine forces. It is a triangular structure when viewed from above, and was crowned by a Dark Energy Reactor. With an unknown height (presumably tens of thousands of feet), it is a fortress that rises both high into the sky and extends a great distance underground as well. The Citadel is located in City 17, and resembles a massive, metallic skyscraper.

The Citadel before its complete destruction.

Half life 2 canals