For the ability to have infinite resources and the only limiting factor being the mind, this is honestly a pretty bad design. The shape is pretty bad in general, especially since the urban areas right outside side the walls provide for excellent cover, and if something were to go horribly wrong and the enemy were to have a break through, they could seize the secondary and tertiary walls very quickly, especially since they’re space marines and they’re extremely quick. Then it’s basically game over because you have just open ground up until the Heaven’s Gate, if they were to go with a sectioned off defense (like Jerusalem in the Siege of Jerusalem of 70 AD.) where it’s more defense in depth-ie than relying on 3 close together walls then it would be much better.