A single Tower of Wizardry simultaneously occupies the same location on both Planes, and can be considered as actually being two Towers that are inextricably linked to one another. Any unit occupying one of these Towers is said to be present in both Towers, on both Planes, at the same time. Such units can essentially choose which Plane to move on when leaving the Tower's tile. A liberated Tower does not "belong" to any one Wizard - any army can utilize it to move to the other Plane. This is why powerful empires will try to block Towers in their area with mighty armies to prevent rivals from using them.