Similar to the Chaos Dungeons and Abyss Raids, the only way to access content in The Tower is to reach the endgame content at Level 50. There are two tiers: Shadespire and Fatespire.
Players are tasked to go into The Tower alone and ascend 50 floors that get progressively harder with various enemies and challenges waiting. Should you die or run out of time, you'll have the chance to make another attempt.
You also have the option to come and go considering the fact that there is no daily cap or catch-up mechanic.
You will need to complete the Public Announcement Tower quest guide before looking going to any populated hub town and searching for the Tower symbol on your map.
Once you interact with The Tower, you will be given the option of which Tower you would like to clear and which floor.
Remember, this activity can only be run by yourself so make sure that you are equipped with good equipment and Engravings before taking on the challenge.