Tower Control is the ability which Aelita Schaeffer (thanks to Annex Program ), William Dunbar (but only has one program created by X.A.N.A. Annex) Odd Della Robbia, Yumi Ishiyama, and Ulrich Stern (because of the source codes implanted in them by X.A.N.A.) possess that allows them to activate or deactivate towers.
Since Aelita is/was connected to Lyoko, she has the ability to activate or deactivate a tower at will by entering the Code: Lyoko, neutralizing X.A.N.A.'s attack in the real world. If she possessed by X.A.N.A., Aelita enters Code: X.A.N.A. on the interface, which automatically deletes the surface sector from Lyoko.
After William fell under X.A.N.A.'s control, X.A.N.A. used William to do his work on Lyoko and therefore gave William the power to control his monsters and assist in the activation of towers. Through William, X.A.N.A is able to activate towers, it is however logical that William has the ability to weaken the resistance of towers which allows X.A.N.A. to activate them quickly as Jeremie had raised defenses on the tower to uphold his clone. William has only used this once in Double Take. It is shown that William has the ability to activate towers by entering Code: X.A.N.A. into the tower interface. He had done this on the Forest Replika to break the team's connection to the real world in the episode Lab Rat.
Eventually, Ulrich, Odd and Yumi have recently displayed the ability to deactivate towers as well, as they had X.A.N.A.'s source codes implanted into their bodies.
As of season 4, Aelita and William are the only ones known to be able to access to the second level of the tower to deactivate or activate it. This was changed in season 5. The process works the same in both activation and deactivation:
1. Enters the tower.
2. Walks to the center of the tower.
3. A virtual elevator lifts the user to the top.
4. Walks to the center of the platform. Upon standing there an interface will appear and she/he enters her hand print in to access the information in the tower.