View down the watchtower on the Great Wall

The watchtowers did not distribute in evenly distance. Some of them were very close to each other with a distance of less than half a mile; some of them were far away to each other with a distance of several miles.
The shapes of the towers were also various. Most of them were square, while some were round or rectangular. And the construction materials of them were different, either. The materials were depending on the local material supplies.