What might have been the purpose of the walls in Great Zimbabwe?

Some experts have tried to come up with a list of plausible uses of the various structures at Great Zimbabwe; The great Zimbabwe walls were called The Great Enclosure or Imbahuru and their purpose were; Defence- that is to withstand any attacks from the state’s enemies; They also served as the residence of the ruling family; They separated the ruling class from the commoners. The Great Enclosure was more luxurious than those in the valley where commoners lived.

i believe the answer is d.