Blizzard
UPDATE: When we originally developed the Legion Timewalking Bonus Event, we were looking for other fun features from Legion we could add to it. Timeworn Keystones were part of that, and the Mage Tower as well.
Having heard feedback and seen how it has played out, we agree that a personal skill challenge being available only for a short window of time every few months isn't in the spirit of these types of events. As such, starting on the week of March 29, the Mage Tower will be accessible at any time, no longer tied to the Legion Timewalking Bonus Event.