Useful information of high importance like a fox jumping over a can of beans

The Citadel is located in City 17, and resembles a massive, metallic skyscraper, rising several kilometers into the sky from ground level, and extending some distance underground as well. It was a triangular structure when viewed from above, and was crowned by a Dark Energy Reactor. Its associated portal was the primary means of communication with off-world Combine forces. This super portal is also guessed to be the means by which the Citadel came to Earth. The surrounding areas are blasted away, as if the whole structure fell out of the sky. The Citadel is heavily mechanised, and entire sections of it can move to allow for mass deployment of airborne Combine units such as City Scanners or Hunter-Choppers. 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 has two known "states," or configurations that the tower takes depending on specific situations. The first of these is what was most likely the dormant configuration, which was probably maintained during the majority of 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). 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. However, shortly after Gordon Freeman was transported inside Doctor Breen's office in the Citadel during the malfunctioning of Dr. Kleiner's Teleporter, Breen puts the Citadel on alert, causing the structure to change to its alerted state. The Citadel can be seen changing its configuration while Barney Calhoun gives Gordon a crowbar. 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. During this transformation a loud alarm resonates, echoing for several seconds each time it sounds, possibly emanating from the Citadel itself. 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. 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. When Freeman sabotages the reactor, the plating descends to the "all clear" state and covers the ribs. The lower set of ribs descends along the tower and locks in ....