The Battlespire was (and it's ruins remain) located in a Slipstream Realm between Oblivion and Mundus, as such a realm allowed mortals to experience and comprehend Oblivion without being driven to madness. More simply, it was possible access numerous realms of Oblivion from the Spire through the use of teleportals and voidgates. The facility was moored in place by five great Anchors, which assured its safety and stability in the surrounding flux. Access to the edifice was granted via the Pillar of Light, a teleportation device linked to the Weir Gate.
The Battlespire was the magical war academy of the Imperial Battlemages, who belonged to the Shadow Legion. The Battlespire has been in existence since at least the Second Era. During the Imperial Simulacrum, Sirran Angada tricked Paxti Bittor into betraying the Battlespire to the legions of the Daedric Prince Mehrunes Dagon.  A pair of trainees became trapped inside; one was captured by Mehrunes Dagon, but the other apprentice followed the trail through numerous Daedric gates to Dagon's citadel, where, by stateing his is protonymic name, they cast the Daedric Prince into Oblivion. Having lost Dagon's support, Jagar Tharn's plot to supplant the true Emperor was doomed to fail.
The Battlespire was partly destroyed during the invasion's aftermath. The invaders were known to wield Daedric Crescents, and succeeded in slaughtering most of those inside. The Empire later reclaimed the ruined Battlespire, and gathered up all the Daedric Crescents to be destroyed, bar one.
The Star Galley Crib
Aside from the Weir Gate itself, the only access to the colleges was via the Star Galley, a magic boat which could navigate the surrounding environs. The vessel was constructed by Battlemage and Artificer Clarentavious Valisious, and was kept in the Star Galley Crib near the Weir Gate, though access was normally restricted.
This section of the facility is also notable for being the home of the gigantic Mock Turtle.
The High Halls and Librarium
The upper sections of the Battlespire consisted of the war college itself. Here were found dormitories, mess halls, libraries, and more. Of particular importance was the voidgate. This device, powered by artifacts known as voidguides, allowed passage to Oblivion. The Soul Cairn was accessed in this manner during Mehrunes Dagon's invasion, but it is not known which other realms the voidgate was able to reach.
Several realms of Oblivion were known to be accessible from the Battlespire or from each other. These include the Soul Cairn, Shade Perilous, the Chimera of Desolation, the Havoc Wellhead, and Dagon's Hunting Lodge in the Deadlands.
Aftermath of the invasion
From "The Chronicles of Janisiere":
"...and the Emperor then commanded to bring forth the two champions who had served him so well, who knelt before him and received his blessing. He bid them rise and said to them, 'Let all present know how these two have served their Empire, for had not Jagar Tharn been robbed of the support of the archfiend Mehrunes Dagon and his fell minions, then Ria Silmane and her champion might never have revealed Tharn's imposture, and long might the rightful emperor have languished imprisoned in Dagon's dark prisons within the Void.' And he then made them great presents of land and chattels, and raised them up among his counselors, and conferred upon them offices of great importance in his Legions."
From "Private Proceedings of the Imperial Commission":
"Reinvestment of Battlespire is neither feasible nor desirable at this time. The open provocation and sweet temptation this installation presents to the Daedric Powers is obvious from recent events, and the installation is of no significant value to the maintenance of the Pax Imperia. In light of recent turbulence in the Border Provinces, and considering more pressing demands upon the treasury, we recommend that the gates to Battlespire be sealed indefinitely."
From "private correspondence of Julianos Cennan, Chamberlain to Uriel Septim":
"The Emperor questioned the heroes closely. One said that, when struck by the enchanted sword, Dagon appeared to have been sucked away and compressed into an ever-smaller volume, and then to have disappeared. The Emperor then inquired among his counselors how long we might be free of the Prince of Destruction. Their answers were various, hopeful, and unpersuasive. 'You have done all that might be done, sighed the Emperor, but Oblivion cannot long hold Dagon, and when he returns, he surely will count us among his most-cherished enemies.'"
According to The Doors of Oblivion, years after Dagon's invasion was defeated, the mage Morian Zenas and his apprentice attempted to use the Weir Gate to enter Oblivion. They found that although the gate still stood and the shattered academy remained accessible from Mundus, the link to Oblivion had been broken. However, this is contradicted by the Crafting Motifs series, according to which the two lived in the Second Era.
- The Doors of Oblivion — Seif-ij Hidja
- ESO Loremaster's Archive — Lord Fa-Nuit-Hen and Tutor Riparius Answer Your Questions
- Events of Battlespire
- Great Anchor warning plaque in Battlespire
- Battlespire opening cinematic
- Elderscrolls.com Codex Archive
- Scamp's dialogue in Battlespire
- Letter to the Grand Marshall of the Imperial Legion — Clarentavious Valisious
- Star Galley Crib Note