A tower is a tall structure, taller than it is wide, often by a significant margin. Considering the origin of the Warcraft series as real time strategy games, the term most often refers to a defensive structure. Using towers offensively against an opponent is known as Offensive Towering.
As defensive structures
In the RPG
A tower is three-level round or square building made of stone. A tall tower constructed atop a prominent landscape feature such as a hill or bluff may be community. The high tower — much larger than a typical watchtower — lets the community keep an eye on the surrounding countryside for a considerable distance, giving it greater advance warning of anyone approaching. The tower community has increased defense, thanks both to its commanding view and the superior elevation over attacking forces that the tower provides the community’s defenders.