The Citadel is the base of operations for the Combine on Earth. The Citadel's primary inter-dimensional teleporter is powered by a dark energy reactor, which explodes at the end of Half-Life 2, devastating the entire Citadel and completely destroying a large portion of the top. Later, Gordon Freeman and Alyx Vance venture into the heart of the Citadel in an attempt to stabilize the construct's primary Dark Energy Core. While temporarily successful, the Core eventually destabilizes and explodes. The resulting blast not only completely destroys the Citadel and City 17, but the energy is harnessed by the Combine, creating a superportal.