Wall in action: https://www.youtube.com/watch?v=QsPaRWpFD1o
I tried to make the simplest and cheapest death-world wall defense I possibly could! Made simple so you can expand and add on as you please. Only seems to take damage if multiple large-scale full-evolution attacks happen at the same time and in the same place. (That and spitters of course. Spitters will always damage your walls)
Add roboports and this should last you forever. :)