Titan Fortress is the HQ of the Empire of Man's military, the UEEF. It's a massive complex roughly the size of Manhattan, stretching from several miles into the ground to just outside of the atmosphere. It includes several large barracks, its own factories, seven underground city blocks for UEEF personnel to live, and a Space Elevator leading to a large orbital platform. This platform is actually the HQ of the UEEF Space Corps.
It's mostly metal structures and prefab buildings, along with miles and miles of tunnels and trenches dug into the ground.
The Titan Accords occurred here