The outer wall of the Great Enclosure is certainly the largest single structure of comparable age in sub-Saharan Africa, but in terms of age and architectural significance the most striking feature of Great Zimbabwe is simply the fact that it is Africa. ... In terms of what drystone construction can achieve, Great Zimbabwe is not well built: the stones were not selected and laid with consideration for their relative sizes ... Even the amount of labour required was not excessive. Presented with detailed plans of the site, a modern drystone building contractor estimated that work force of 84 men on a six-day week could build the entire complex in two years.
The walls look in danger of being pushed over by any attackers. So, what was the point? In general, I have a hard time seeing why you wouldn't build defensive structures, but, apparently, large swathes of the Earth didn't see the point.