Whether or not the Dark Tower appeared after the Prim receded or if it has always existed is unknown.
Humanity in its great ignorance attempted to toy with the Tower in some way, form, or fashion, and managed to unbalance it, requiring support beams to hold it up.
The beams are beams of force that run through all realities. There are Six beams that are guarded by twelves guardians, with a single lost guardian at the top.
If the Tower falls, then reality recedes like a scroll, ending instantaneously all of existence.
May only be entered by one carring a sigil of Arthur Eld.
See the World Tree, Yggdrasil, Axis Mundi.