Towering is one of the many activities you can do in Graal. Towering is very simple, just attack or defend the flag. A lot of strategies come through towering. It's different for every guild and different for every tower.
(Note, since the in-game map from 2012-2013 has been updated, York Tower and Snow Tower no longer exists in the game overworld until further notice, and also on side note, the map image above is outdated so there is no image of the location of Destiny Tower.)