Maethrillian

The Capital was the political center of the Forerunner Ecumene. It was a massive structure, exceeding the scale of even the Halo installations.

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.

The Capital was home to the Council amphitheater, where the Forerunner 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.

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.

Trivia

 * The Capital bears some similarities with the Citadel space station in the Mass Effect series: Both are planet-scale artificial stations featuring inhabited cities, both serve as a center of galactic politics, and both are able to change their structure to "close" themselves in the event of siege.

List of appearances

 * Halo: Cryptum