

When Freeman sabotages the reactor, the plating descends to the "all clear" state and covers the ribs. The Citadel can also be seen from the rear near the first gate and "Water Hazard", showing a dull pair of metal strips separated by one vertical white beam. A small piece of plating barely visible on the left (southern) side of the Citadel also descends partly down the tower, scanners and other ships 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 teleport, Breen puts the Citadel on alert, causing the structure to change to its alerted state. However, shortly after the player is transported inside Doctor Breen's office in the Citadel during the malfunctioning of Dr. This configuration is 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 is most likely the dormant configuration, which is probably maintained during the majority of life in the city, when no serious threats are posed to the Combine (the structure also takes this shape during Half-Life 2: Episode One).

The Citadel had two known "states," or configurations that the tower takes depending on specific situations. The Citadel is heavily mechanized, and entire sections of it can move to allow for mass deployment of airborne Combine units such as City Scanners. The surrounding areas are blasted away, as if the core of the structure fell out of the sky. The Citadel likely came to Earth via a Superportal. Its associated portal is the primary means of communication with offworld Combine forces. It is a triangular structure when viewed from above, and is crowned by a Dark Energy Reactor. It was located in City 17, and resembled a massive, metallic skyscraper, rising approximately 2.6 kilometers (8430.9 feet tall) into the sky from ground level, and extending some distance underground as well. The Citadel was the headquarters from which the Combine govern the Earth.
