Theory time...
The Imperial Battlespire is an academy which trains imperial battlemages so that they become acclimated to being outside of Nirn. In other words, mananaut training. The Battlespire is in Aetherius, and I see no other way for it to have been constructed there than it being built on Nirn and moved into Aetherius, in other words, it is also a spaceship. Judging by the dates, the Weir Gate and the Battlespire have existed at least since 2E 582, perhaps even before that, so I propose they were built by order of Reman I, who was known to be interested in expanding his empire further than Tamriel—he wanted his empire to stretch across the stars. The Mothships likely required precision and finesse to grow and train, but the Battlespire being a non-organic construction didn’t, hence the Battlespire was finished before the Mothships, and it was the Remanic Empire’s first true voidship launch, proving useful for mananauts decades, perhaps even centuries, to come. There would of course have to be some way to train mananauts to be used to void travel, without experiencing the mental decrepitude often found while traversing Oblivion, and the Battlespire was the perfect place for it. The Imperial Battlemages were likely closely related to the mananauts, perhaps even being one and the same, considering both were trained at the Battlespire. After the space expeditions were shut down, the Battlespire shifted into Battlemage training, but I theorize it used to focus even more on the mananauts. The Battlespire was a Remanic-era spaceship academy for training mananauts.
