The Imperial Battlespire was an academy for training of the Battlemages of the Imperial Legion. The Battlespire was moored in its place and sustained by five anchors, with the Battlespire itself appearing to rest at some great altitude without any physical base. From Tamriel, the Battlespire was accessed through the Weir Gate, and from the Battlespire, it was possible to access the various planes of Oblivion.
During the Imperial Simulacrum, the Battlespire was captured by the forces of Mehrunes Dagon. Dagon had also, during his siege of the Battlespire, intruded upon the Shade Perilous (under the control of Nocturnal) and the Soul Cairn (under the control of the Ideal Masters) as well as having forces within the Havoc Wellhead and the Chimera of Desolation.
During the course of the events depicted in An Elder Scrolls Legend: Battlespire, the forces of Mehrunes Dagon were repelled from the Battlespire at the cost of the Battlespire's destruction. While the shattered remains of the old Battlemage academy can still be reached through the Weir Gate, it is no longer possible to traverse to and from oblivion via the Battlespire.