Maethrillian

Maethrillian, or the Capital, was the political center of the Forerunners in the Halo Universe. It was a massive structure that exceeded the scale of even the Halo installations. It 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. 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.