[![enter image description here](https://i.stack.imgur.com/9qelv.jpg)](https://i.stack.imgur.com/9qelv.jpg)

This is a basic map of a castle in an alternate Earth. It was built with two walls, both of which were always built in a circle, with four broad, circular towers/gatehouses built in four cardinal directions—north, south, east and west. The outer wall protected the towns owned by the noble and gentry classes. The inner wall guarded the palace, which, on this alternate Earth, looked more like a mansion along the lines of Harlaxton Manor in England or Versailles in France than the many towers and turrets of fairy tale imagery.

The outer wall had to be barricaded by a moat, dug from a river and with the right width and depth to deter invasion, and the inner wall had to be built at the foot of a hill.

Could this sort of defense mechanism be feasible in real life?