Hard to say. The Ceporah Tower is older even than the High Elves' arrival on Summerset from Aldmeris. Its interior reminds me of a big snail or clam shell, and water and fossils also seem to be strongly associated with it, too.
I'd venture a guess and say either it's an Ehlnofey (Earth Bone) body or an old Sload structure of some kind.
And about its origin...
You will find out by doing the Tutorial quest and joining the puzzle pieces, between the architecture in the final part of the tutorial + Cloudrest last boss + architecture of Ceporah