Half-Life 2 The Citadel (2004) Version

Citadel
The Citadel is a gigantic Combine megastructure located in the heart of City 17, which serves as the main base of operations and primary headquarters of the Overwatch, from which they govern the Earth. Easily the most striking symbol of the Combine's rule, the Citadel looms ominously above the city as a constant reminder of their presence, and serves as the personal residence of Earth's appointed Administrator, Dr. Wallace Breen.
Overview
The Citadel is located at the epicenter of City 17, and resembles an enormous metallic skyscraper thousands of feet tall. A true monolith, 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 Fusion 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.
The Citadel is completely surrounded by enormous Smart Barriers which protect the structure from would-be attackers attempting to breach the walls. Attached to the exterior walls of the Citadel, within a huge chasm surrounded by broken sewer pipes and destroyed vehicles, are large pieces of machinery that dig the ground, possibly expanding the underground area to allow for further construction. These machines are also theorized to function as giant Restrictors to repel Antlions, though such theory is unlikely as the vibrations could not cover the entire city. The game files also mention the machines as "Diggers" instead of thumpers.
The Citadel has two known "states" or configurations that the structure takes depending on specific situations. The first of these is the dormant configuration, which was most likely maintained throughout the majority of the Combine's rule, when no serious threats were posed. This configuration is characterised by a single set of rib-like structures exposed within the rightmost half of the Citadel, with an outer metal plating retracted slightly above an inner plating. However, shortly after Gordon Freeman is transported inside 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. Barney Calhoun notes that this is a rare occasion, stating that he's "never seen it lit up like that." The alert configuration involves the inner plating retracting upwards to reveal a total of four sets of "ribs", as seen at the start of Route Kanal whereupon the reconfiguration has fully completed.
During this transformation, a loud alarm is heard resonating throughout the city, possibly emanating from the Citadel itself, and Scanners are seen rapidly deploying from the structure's midsection to hunt for Freeman. The Citadel remains in this configuration for the remainder of the game, only changing again during Episode One after Freeman sabotages the reactor, with the plating descending once more to the "all clear" state, covering the ribs. It can be theorized that there are other configurations the Citadel may take in different situations, but these two are the only configurations depicted in the games
The book Raising the Bar mentions that entire blocks from City 17 vanished into thin air as the Combine delivered the Citadel to Earth. However, this was likely changed during development as the game shows no signs of such an event. Half-Life: Alyx shows the Citadel under construction, confirming it was built by the Combine after the Seven Hour War. Based on a drawing in Alyx's apartment, only the Citadel's Core was teleported to Earth as single spire roughly the same height as the completed Citadel, with the rest of the architecture constructed around it.
The existence of more Citadels located around the world is mentioned in-game by Dr. Kleiner and is also mentioned by Eli in The Final Hours of Half-Life: Alyx. The number of Citadels on Earth still remains unknown, as do whether they all bear the same height and general appearance