Also called The Spire, The Twilit Fastnesses, or the Celestial Citadel; the Battlespire is a void-castle that served as an elite magical war academy for the Imperial Battle College and Imperial Battlemages. It was a testing facility for those seeking to join the Imperial Guard.
During the Imperial Simulacrum (c. 3E 398), a large contingent of Daedra invaded the Slipstream Realm as a whole, and took control of the Battlespire, severing the connection of the gates between it and the Prime Material Plane.