In reality, it still exists but only shows itself to certain people and contains many magic portals through which one can travel to many different worlds. It's further believed that only chosen ones could command the tower to open a portal within, giving it the moniker "Gate of Worlds" and "Threshold of Time".
History
In late 1267, Ciri, believing she could use the portal within to take her back to Thanedd and far away from those hunting her, made haste for this tower, eventually entering, but instead of going back to the island she entered the elven world of the Aen Elle.