Citadel

The first appearance of the Citadel in Half-Life 2.

The Combine Citadel as seen in the intro of Episode One, crippled after the destruction of its Dark Fusion reactor at the end of Half-Life 2.

Citadels are mega structures constructed by the Combine for the sole purpose of governing the Earth. The only citadel actually viewable by the player is located within City 17, and resembles a massive, metallic and largely featureless skyscraper, stretching high into the clouds. The Citadel also extends deep underground as it digs to devour resources. In the Seven Hour War, it is now known that the Citadel in City 17 and most likely all of the other major cities around the world were transported and landed on the spots they reside on in the present day. From here, the Combine could launch their endless attacks on the world's forces from within their well protected cities (since most of the military were forced to hound all civilians into the cities and fortify them because of The Portal Storms).

The Citadel's outer wall is dark blue and contains a Razor Wall around its perimeter that is constantly moving, devouring the city and assembling itself into whatever it has been configured to be. Within the Citadel itself is a mass complex of factory machines, computers and rails used to transport 'pods' containing Stalkers or prisoners. With the Citadel continuously digging underneath to gather resources, it can create an nearly unlimited supply of Synths.

The entirety of the Citadel covers the entire landscape of the city, and can be seen at different points during Half-Life 2, the purpose of it shadowing the city like it does is to remind its remaining inhabitants of who is charge. As Gordon Freeman and Alyx Vance steadily make their way out of City 17 in Episode One, the now crippled Citadel can also be seen at different points of the city.

At the very top of the Citadel is the Dark Fusion Reactor, capable of teleporting matter to an outpost in the Combine Universe (presumably another Citadel). In the final events of Half-Life 2, Gordon Freeman purposely destroys the dark fusion reactor, foiling Dr. Breen's plan for escape. However, the destruction of the reactor causes massive explosion on top of the Citadel, severely damaging the structure's fusion core deep within its bowels, and eventually causing the whole 'network of linked Citadel reactors to fail completely' (as quoted in Dr. Isaac Kleiner's Kleinercast). This meant that matter could no longer be transported between the Citadels on Earth and within the Combine Universe, eventing in a huge weakness in the trans-human and Synth Combine forces.

In Episode One, Alyx Vance and Gordon Freeman reappear at the foot of the Citadel, which was then incredibly unstable and containing a swirling fusion vortex above its tip, raining debris and ash to the ground below. They are both forced to enter the Citadel again, where Freeman temporarily stabilizes the fusion core, giving Alyx, Gordon and whoever was left in City 17 time to escape before the entire structure explodes, wiping out the entire city with it.

Whilst doing this, Alyx Vance also finds out that the remaining Combine forces inside the Citadel at the time, were trying to overload the core themselves, as a means of using the incredible power output to send a very important transmission to the Combine Homeworld. Alyx and Gordon witness the transmission being sent as well as the Citadel's destruction, as they speed away from City 17 on a train. Several escape pods are also jettisoned before its destruction, and fly past the duo on the train. The destination of these pods is unknown, although based on the psychic attack received by Gordon and Alyx on the train ride out of City 17 and further encounters in Episode 2, they are Combine Advisors.

Following the City 17 Citadel at the end of Episode 1, the remainder of the citadel begins to emit powerful 'portal energy' into the swirling fusion vortex which hovers high above the citadel. This energy is specifically released to help gain enough power to once again open a portal connecting to the combine overworld. As Gordon Freeman awakes from the derailed train cart in which he narrowly escaped the cities destruction, he and Alyx Vance witness sections of the citadel scattered throughout the ruins of the city. The collapsed citadel now fires a large blue electrical ray of energy straight from the core seen in Episode 1. The energy then spurs a large Portal Storm, a massive amount of energy fired out in one burst which spans out in multiple directions. These portal storms can cause massive damage to structures and landscape, as witnessed by Alyx and Gordon at the very beginning of Episode 2. What remains of the citadel is unknown, it is unknown whether the core is still intact and is purposely sending the energy out, or whether it was simply the combine's plan for the core to override with energy. The portal is shut down by Dr. Magnusson's rocket moments before opening and beginning another combine invasion.

The citadel is unlikely to be seen again in Episode 3, as the locations are many miles further away from the city than White Forest and it's resistance base.