The barrier shielding the tower can only be opened by activating the imbued gems in the correct order. The order is only known to Xylem and his inner circle of apprentices. No others have yet been able to decipher the code, and mistakes are met with deadly consequences.
The aspiring apprentices spend a lot of their time around the tower. Some may have seen or heard something that might point to a solution.
Let us hope, for your sake, their information proves valuable.