The corner towers were defensive towers built at the corners of castles or fortresses.
Two ideas have been advanced about the purpose or value of corner towers in medieval fortresses:
Towers constructed at fortress corners were larger and taller than other towers. At the bottom of these towers were defences, such as ditches, fences, and sometimes advanced forts or bastions. 
Corner towers may be seen on the Wall of Philip II Augustus (Tour du coin (Louvre), Tour de Nesle, Tour Barbeau), on the Wall of Charles V (Tour du bois), in the city of Carcassonne, in the Château de Pierrefonds and in the fortress of the Bastille.
In architecture of non-defensive structures, like churches and theater buildings, a corner tower is any tower that is protruding upwards from the corner of two walls, and usually has no walls of its own below the roof. While other towers are usually attached to the building by one wall.