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.
Differences From Canon
In this version of Ar Tonelico, Lyner and company failed to redeem Mir after awakening Shurelia. Mir successfully hijacked the Tower from Shurelia, becoming the new de facto administrator, and ruled with an 'iron fist' for quite some while. However, with the help of a group from the Multiverse Mir was able to implement a plan to save Ar Ciel from destruction, causing the Sea of Death to recede. Her attitude somewhat softened, she now rules a segregated community with Reyvateils at the top of the tower, and humans working the fields at the base, with an experimental 'integrated colony' on the remaining Wing of Horus.
Infel also has a physical body in this theme, and controls the Second Tower. Frelia is awake and peacefully living in Sol Marta.