The world of Ar Ciel is one in which sound - and more importantly, music - holds great power. Fundamentally, its world is split into two energies: the 'spiritual' of H-Waves, and the 'material' of D-Waves. Long ago, humanity learned to harness the relationship between these two, converting H-Waves into D-Waves and enabling manipulation of the world itself in a manner much like magic.
To do this, humanity created several tools. The first were the Towers, three gigantic structures that scrape the heavens themselves and channel energy from the core of the planet, into space, and back again. The second were the Reyvateils, genetically-constructed females who could sync with these Towers, called Song Servers, and direct the energies to manifest in neo-magical effects. Technology itself relied upon this relationship, as well, with energy from Symphonic Reactors powering houses and so forth.
Yet at what was to be the height of their triumph, a crisis occurred. The flow of energy between the planet and the Towers was distorted, and the world fell into ruin, covered by the destructive and vicious Sea of Death. Now Ar Ciel is dying, and the remnants of humanity and Reyvateils both cling tenaciously to the Towers, the only livable places upon the planet.
At the current time, none of the three Towers are controlled by their original Administrators. Two are controlled by automated maintenance programs(Raki and Harvestaysha), while the First Tower is now controlled by the 'Virus' Mir.