Maethrillian

The capital was the political center of the Forerunner ecumene. It was a massive structure, greatly exceeding the scale of even the Halo installations.

Overview
The capital was one of the oldest structures in the ecumene, having been constructed roughly around 125,000 BCE. It housed a population of several hundred thousand Forerunners, mostly high-ranking Builders. Trillions of ancillas served in the capital in the capital metarchy, an enormous network coordinated by a Metarch-level ancilla that answered to the chief councilor.

It was one of the most trafficked locations in the ecumene, with constant security patrols, sentinels, and transports swarming around it. Within five thousand kilometers of the structure, incoming vessels would be carefully screened by sensory fields and directed by hard light docking nets. The capital's system housed multiple slipspace portals that linked to many worlds, as well as the Ark.

Design details
The capital was composed of a series of perfectly circular "platforms" arranged in a spiral along a central rod. These platforms decreased in size toward either end of the rod, with the widest central one being approximately 100,000 kilometers in diameter. Each platform was covered with a "city" of structures and buildings, whose gravity was controlled individually by buffer fields.

The capital was home to the Council amphitheater, where the Ecumene Council engaged in debate on important matters. The amphitheater was suspended in midair, attached to the rest of the capital by bridges, anti-gravity gondolas, and lift tubes for individual Councilors. During Council proceedings, the amphitheater would be decorated with display spheres showing representations of the twelve of the systems inhabited by early Forerunners, each carrying an epistle of the Mantle. Near the Council amphitheater there was a hallway with its walls decorated with countless spent slipspace flakes, some of which had also been built into sculptures.

Defenses
In the event of a large-scale attack, the capital could rotate its sections to assume a spherical shape, thus protecting the structures atop each platform from damage. It also possessed powerful energy shields and a large defensive fleet. If the capital metarchy coordinating the fleet was compromised, the fleet would splinter into individual squadrons, each having specific instructions on how to deal with an attack.

Trivia
The capital is similar in design and function to the Citadel space station from the Mass Effect series. Both are large-scale artificial stations featuring inhabited cities, both serve as a center of galactic politics, and both are able to change their structure, "closing" themselves in the event of siege. However, the Citadel is only approximately forty-five kilometers long and thirteen kilometers in diameter, as opposed to the scale of the capital, which is closer to that of a gas giant.

List of appearances

 * Halo: Cryptum
 * Halo 4