The S.P.U. is the second highest point in the Tower of Ar tonelico, located right above the Prism Garden and right below the Rinkernator, and it's the facility that houses the Mainframe of Ar tonelico, the supercomputer that processes and maintains the Binary Field of the Tower and all of the SH Servers connected to it. Its acronym stands for Song Processing Unit, and due to its great importance, it's protected by an extremely powerful shield. It's the final area that can be accesed through the Dividing Gates, given that the highest sector of the Tower, Sector A9, has its dividing gate come out into this place.