Location from Half-Life 2 by Valve
The Combine's fortress-reactor at the heart of City 17 — a dark metal spire miles high that houses their administration, their army, and a dark energy core powerful enough to tear open dimensions.
The Citadel is the Combine's primary infrastructure on Earth — administration, military staging, communications relay, and dark energy reactor all in one structure. Standing at its base and looking up, you cannot see the top. It generates its own weather. It was not built so much as deployed — the Combine dropped it into City 17 as a prefabricated power projection platform. Inside, the Citadel is a nightmare of efficiency. Combine technology processes, sorts, and converts. Stalkers — humans reduced to cybernetic husks — maintain the machinery. Overwatch soldiers stage from internal barracks. Advisors float through sealed upper chambers. The dark energy core at the center powers everything and serves as a dimensional communication device. The Citadel's destruction at the end of Half-Life 2 — and its slow, catastrophic meltdown through Episode One — drives the evacuation of City 17. Its core explosion creates the superportal that threatens to reconnect Earth to the Combine Overworld.
An impossibly tall structure of dark metal and alien engineering, rising from the center of City 17 like a needle puncturing the sky. Its surface is not smooth — it's a lattice of struts, conduits, and moving mechanical components that shift and reconfigure. Internal corridors are vast, cold, and lit by harsh blue light. The dark energy core at its heart is a swirling sphere of contained energy, beautiful and terrible, visible through observation platforms where Breen delivers his broadcasts.
Also known as: The Citadel, Citadel, The Tower, Combine Citadel