In brief
The White Tower is the core of a fortified complex with many historic layers.
Useful to know
The White Tower was a Norman military structure erected under William the Conqueror near the Thames. The fortified complex known as the Tower of London developed around it. UNESCO has linked the construction to both protection and a show of power. [1]
On the plan, separate the central volume from the surrounding fortifications. This helps to understand why the singular name does not describe the entire object. The various belts and structures are to be regarded as a developed system and not as a one-time completed self-contained tower.
Sources
For further detail and to check the information: