🤔mmhhh what do you mean with sandbox?
the tower of wisdom can be unlocked after chapter 5, when you defeat enilyn himain and join her to the party, she will provide you access back to the magic academia, and you can start the side-quests.
Also, if you attempt to open the tower before enilynn, you will open the side-quest back to the school, which will be updated and completed upon you join enilynn and go back to the tower.
It has been introduced as a generic side-quest content, hopefully to expand the story at some point, but attempting to stick to the lore of the original main quest, therefore, there's no way in this release to gain access to the tower atm before meeting enilynn.