Taking the flag and PKing. The two things you can do. I do not group them as one because:

PK
-Purpose: To kill other players.
-Involves a single player acting on his/her own accord.
-The tower is a good place for PKers because of the amount of players gathered in one place as well as being able to take advantage of the chaos.
Tower Taking
-Purpose: To capture the flag and to defend the flag.
-Involves a group of guild members acting on a leaders command (or a person who really thinks they can take a tower by themselves).